Injustice, because they refuse to see the other side of the coin, or the full picture, and a decision ignoring the full picture can be just only by accident (at least in common moral systems).

Stupidity, because it betrays a lack of touch with reality. In this regard I am reminded of a recent thread here, where an article about artificial meat was being discussed. An author of the article expressed their uncomfortable feeling when eating the artificial meat, caused by their thought about the unnatural, "lab" origin of that substance. If that article was sincere, that person have probably never visited a slaughterhouse, or killed an animal by their own hands in order to eat it. This kind of cessation of contact with underlying reality behind our everyday experience is quite common in our modern societies.

In order to be succinct, let me just say that historically it was common for elderly people to live in multigenerational households, sharing their space with a younger family and helping (as long as they could) with chores and taking care of children. While that is not the only viable way, and I am in no way against the modern pension system in general (although its current implementations in most if not all western societies have their problems; and also make many people to lose "touch with reality" in the way I alluded to above), it was quite a natural way of handling the issue of old age that reflected constraints under which a human society existed (and still continues to exist today).

Since I feel that the dominant sentiment of the commenters here is based on emotions and "morality" in a way that I believe is inherently evil, I will close this comment with a refrain from emotions and morality, and state a cold-hearted truth: A society that discards its old people might be a bad one to live in but might be able to survive. A society that discards its young people is destined to die. [To avoid misunderstanding: I do not argue that we should discard old people.]

For all practical purposes property/ownership is a service provided by society, not a static "attribute" of stuff (the only exception is when you live entirely without any sort of contact with other people, in which case, however, the concept of property becomes void, at least practically). And any service carries a cost with it.

For instance, in our kind of societies, the institution of property requires, at the very least: (1) legislature that defines what property is and all the related rules, (2) the administrative part that handles property transfers and other issues, and keeps the record of who own what, (3) the judiciary system that interprets the legislation and resolves disagreements and other issues related to property, (4) the police or some other body that enforces and protects the whole system within the society, (5) the army that protect the property of the society against the claims of external groups. In our societies many of these institutions carry also other functions, but enabling the system of ownership is an important part of their job. And this service is very expensive --- so not paying any taxes on property means that those costs must be covered by some other sources (e.g. selling natural resources, income taxes, ...) and, usually, lead to a transfer of wealth within the society.

So the idea that you just "own" something (your house for instance) and any kind costs of that imposed on you by the society/government are unjust is completely and patently misguided. If anything, the opposite it true.

(That does not mean that the tax system must be necessarily constructed to reflect that, as there is no single and obvious correct way to design a tax system, and other important considerations enter that issue.)

A year or so ago I completed a PhD in theoretical high energy physics. I did that after working for about 5 years outside of academia [the reasons for that were financial issues in my family]. Because of this hiatus (I suppose) and the fact that I had troubles getting reference letters (my master thesis advisor passed away at quite a young age right at the time when I tried to start applying for positions; we actually agreed to meet in person to discuss next steps but the meeting never happened) I had issues getting accepted into PhD programs and I ended up with an offer from a rather weak institution in my home country (which is in EU). I had no other options to choose from (and I could not wait another year) so I accepted. However, I also looked at the publishing record of the team that I was about to join. Their works did not seem stellar (and I did not expect that) but they seemed to publish regularly, on several topics, and with several collaborating institutions.

When I started I quickly realized that the knowledge/expertise level was much lower than I expected (and I did not expect too much). The group consisted of the group leader and three senior researchers, and I am pretty sure that my knowledge of QFT when I *started* working with them was the best in the group. (Now, I had studied in my own time during some part of the years I was outside of academia, and I think my knowledge at that time exceeded that of an average 1st year hep-theory PhD student in Europe, but it was also nothing spectacular; definitely not what I would call "competent"; which is admittedly a high bar in quantum field theory / hep theory, but something I would expect senior researchers to more or less satisfy.)

I realized that the one topic on which the group was publishing on their own was just a continual rehash of the same thing (applied to various different problems, so perhaps not completely useless) and the other topics, those which seemed more advanced when I originally looked at the publication record, were all done essentially outside the group by other people. The group members contributed with some non-essential help (often resembling a work done by a student, such as finalizing a manuscript or double checking calculations) or with nothing; and were added as coauthors due to some other reasons (I suppose: past loyalties, friend groups, advantage of having foreign or external institution co-authors). [Note: I was not included into any of those collaborations so I am not speaking with a full knowledge of the inner workings there.]

During this time I saw that people being added as coauthors had a very noisy relation to what they did or didn't do on any given paper. I was added as a co-author on a paper that I made essentially no work on [not fair], I was added as a co-author on papers which drew upon some of my earlier results [fair, but I did not subscribe to the overall spirit of the paper or even to the paper being published at all], I was a co-author of papers where number of authors and my positions in the list roughly corresponded to my contribution [as it should be], I was a co-author of papers that were done basically by myself but there were several other co-authors included who made a little to no contribution [and the ordering of the author list was always alphabetical not reflecting the relative contribution].

There is also another aspect of this: If you are an early career researcher (PhD student, PostDoc, and even non-tenured professor) you might have a very limited control over who gets included on the publications you work on, and on what publications your name appears. In my case, I am a very disagreeable person when I think things are being done wrongly and yet I have not managed to refuse from being included on papers I did not want to be a co-author of, or to prevent people who contributed nothing to be included on my papers. [I mean the only way to achieve that meant escalating conflict into levels which (a) would likely disable any further cooperation with the group, and (b) might be even morally questionable concerning the level of distress it would make to other people who just seemed to be fine about "the normal way" things were being done.]

In any case, while there are still some really good people in academia (actually the best people I met in my life were nearly all in academia), in overall (by no means fully indicated in the above paragraphs) I feel very bitter about it, and I think it is so dysfunctional and morally corrupted, that it is nearly inevitable that it collapses in future. The current and future shockwaves of the change in the public sentiment and funding, the evolution of demographics, and the consequences of AI, just hasten the process that would have probably unfolded, sooner or later, anyway.

Your points about Leibniz are very interesting, and while I find that line of thought fascinating I must admit my own lack of knowledge of the historical context. In any case, thank you for pointing out that idea!

(1) The relativity principle is exactly what you refer to yourself. You can't apply "Landau--Lifshitz"-like arguments without it. (And I don't think these principles count as medieval knowledge.)

(2) I mentioned electromagnetism, because you need some clue for the concept of an absolute speed that is same for all inertial observers. This is very counterintuitive from our everyday experience, and counterintuitive from the point of view of somebody living in the time of "Galilean" or Newtonian mechanics. Theory of electromagnetism is the only thing that I am aware of, that is nearly (by a stretch) accessible at the level of "medieval" knowledge, from which the concept of constant speed follows. (Historically: Maxwell's completion of previously inconsistent equations of electromagnetism yielded a wave solution propagating with the constant speed of light. This was interpreted in terms of the ether originally, but it is a strong hint in itself for Einsteinian principle of relativity; and a reasoning along the lines you alluded to can them be applied, at least in principle.)

Perhaps it would be possible if the classically observed reality turned out to be possible (consistent under some reasonable conditions) only as a consequences of some deeper, sufficiently determined theory; but that would go far beyond our current level of knowledge, at least as far as I can say.