If you had never read a book before, and someone was trying to convince you to try it, what could they point to that would fulfill all your criteria? Would it be enough to say it makes you smarter? That's not very specific. It sharpens your thinking? Makes you more empathetic? That would all seem like 'vague undecipherable gibberish' if you had no experience with it. They might resort to saying that it can connect you with a great dialogue that has been occurring for over two thousand years, but as you say, the fact that people have been doing it for thousands of years doesn't make it interesting or valuable.

Seeing a study that some part of the brain responds more quickly for up to 90 minutes after reading or that people with gardens live 0.28 years longer on average would not make me want to do those things more, because those are NOT the benefits of doing those things. You have to figure out what you're supposed to do with your one human life. Science is one tool, culture is another. Neither of them makes the other superfluous.

I'm curious, do you think it's an order of magnitude too low or too high?

https://xkcd.com/1162/

If a company says they've built the fastest car in the world, a reasonable response is "ok, let's see it drive faster than any other car can", even though they already said that it can do that.

...yes? If I go to a website called "_ News" (present company included), I expect to see either news stories aggregated by humans or news stories written and fact checked by humans. That's why newspapers have fact checking departments, but they're being replaced by something with almost none of the utility and its proponents are framing the benefits of the old system as impossible or impractical.

So his background in economics probably got him into an economics PhD program.

I don't think Stallman is abrasive out of a sense of respect and duty to the system of public debate.