Old Java threads were 1:N, not M:N, and were instead of rather than in addition to support for 1:1 native threads. Having the choice between 1:1 and M:N both available on the platform is better than being locked into 1:1 or 1:N (or even M:N.)

What is offered here is strictly better than the current or previous state.

> when folks realized that OS-native threads are the way to go if you have them.

Well, it depends on what you're trying to do. Having the option of using either is preferable.

Absolutely, especially if the lightweight threads are implemented on top of OS threads, which is not what was done with Green Threads.