Mistyped, it's sealed interface.

> So you can always have `Maybe<T> x = null`, or even `Some<T> x = null`.

Yeah and? Practically every type system have escape hatches, like Haskell can also do side effects without the IO monad, does it make the latter useless?

The whole point of using Optional/Maybe is to prevent the possibility of accidemtally creating nulls. If you don't make mistakes, then nullability is not a problem. If you do make mistakes, then a class that only helps when you don't make mistakes is basically useless.

This also has significant impact for serialization/de serialization - a classic place where you get unexpected nulls, that Java Optional/Maybe don't help with at all.