Streams are not a language feature, they only depend on generics and lambdas. There are surprisingly expressive FP libs available for Java written with only existing tools (vavr for example).
Also, Java deliberately move slowly, it is a huge benefit for the platform as it can better avoid potentially vain features that would have to be maintained forever even if they are not popular. Algebraic datatypes debuted many many decades ago, yet they only become sort of mainstream in the last couple of years.
Both lambdas and generics did not ship with Java originally. Yes Java moves slowly, but it would move even more slowly if they waited for published studies proving the productivity gains of these features.
Also, Java deliberately move slowly, it is a huge benefit for the platform as it can better avoid potentially vain features that would have to be maintained forever even if they are not popular. Algebraic datatypes debuted many many decades ago, yet they only become sort of mainstream in the last couple of years.