[lmm]

But there's no reason Java threads have to be heavy. Flip the question around: suppose you had a good syntax for nonblocking logic, and a library ecosystem where everything was async by default. In such a context (and we're getting there), why would you ever want to use traditional 4k threads? The only answers I can think of are for heavy number crunching and just maybe I/O on very fast dedicated devices.

So I think that's the future we're looking at: most code async, with the likes of LAPACK and some ultra-low-latency I/O libraries (hedge funds and the like) being the only users of traditional heavy threads.