"is there any reason to think that in 30-40 years the only people writing Java will be retirees maintaining old banking systems?"

I don't think so. But it's pretty much guaranteed that a lot of the people who are complaining about COBOL today are writing systems that will be legacy in 30 years. And the generation of programmers then will be complaining about today's programmers.

Especially when I look at node or python with tons of external packages (.NET going the same way), I don't see a good long term future.