Every language is fundamentally broken from day 1, whether it is a known tradeoff or something that remains hidden for several years.
Also, even if you had the perfect language with a presumably not-mainstream feature, you would have to make that feature somewhat known in the developer ecosystem (just look at Rust’s borrow checker, it does cause issues for newcomers as opposed to features already having an analog in other languages). While FP ideas were known before Java (ML predates it by quite a few years), it has become only recently more acceptable, so it makes sense to only know incorporate it into the language (and yet again, do note that FP is not a silver bullet so java has to let developers write imperative constructs just as well as before)
Unlike first-class monads, sum types are not some sort of esoteric FP construct that arises from challenging technical problems around dealing with external state. They are the very obvious dual of product types, which every language has some version of. They are easy to understand, require very little tutorial-ing, and their power is immediately obvious when you start using them.
Product types model "x and y", sum types model "x or y". One of them is universal. The other has been totally missing from mainstream languages until recently. No wonder our software sucks so much.
I agree with you in that I really like and really miss them from languages since most of them doesn’t have it in a feasible way. But sealed classes are coming fortunately in Java at least.
