Except I don't think you can just magically do that. People do this in Python, but they do it in Python because 1) Python is dumb and has the GIL, much easier

Except I don't think you can just magically do that. People do this in Python, but they do it in Python because 1) Python is dumb and has the GIL, much easier to hack in 2) They use reflection to change libraries, but it breaks anytime you would hit native C code. With Java trying to JIT everything, you are now having to change threading at the assembly level. I think it is would be a ton of work to do at the base JVM level. If you look at the people behind the library I linked, it is many of the core Java people. They might have more insight on reasons why it won't work.