Regarding language syntax, it definitely matters. In the same way the vocabulary we use shapes our thoughts, the expression of a programming language shapes the implementation. Of course, as Clojurists know all too well, it's entirely possible to write Java in any language!

IME development speed is a very minor factor in the success of a vertical SaaS. Vertical niches exist because they are experts in something other than software, and understand it's worth paying for their problems to be solved. Typically, subscriptions of successful software businesses are priced based on outcome/value, not the cost of development.