LLM bashing aside (although I tend to agree), I agree with midnight_eclair. The claim that Erlang or Go are outright superior to the JVM doesn't really stand up. They're better at some things, and worse than others.
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!
I don't know if I agree with your line of thinking.
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.