What I find funny is Java started with green threads, then moved away to system threads, then back again.

Maybe massive concurrency was not that big of a feature back then. But these days everyone wants to support a million connections at a time. Green threads and async tasks can do that without breaking a sweat, unlike OS threads. Also, Java virtual threads are still cooperative. Maybe they will move to preemption in time like Go did.

Some time ago I tried to run just 10k OS threads on a small PC and it just crashed. So clearly OS threads have not improved much.