I dont think it is inertia, it is just that making stacks small enough is a harder issue than just making a fast evented system. More to change. Will take longer or involve more radical change, like shifting to Erlang say...

Fundamentally, this is a compiler and runtime issue. If they guys behind some mainstream programming language wanted to, they could go and add lightweight threads that people could just use. It would take a lot of engineering effort, and there would be some compatibility edge cases, but it could definitely be done.

(In the meantime, I'm happily using eventlet in Python, and some people swear by gevent. These have their shortcomings, but they're worth the trouble, easily.)

Ruby 1.9 has lightweight threads built-in (fibers) but you have to write and manage the scheduler on your own. That also means you can't use any existing IO libraries that already know how to yield back to the scheduler, but it's only a matter of time before that's a solved problem.

Nice! The big caveat here is that they're cooperatively-scheduled, and they've left the I/O stuff up to libraries. In other words, they look exactly like Python's greenlets, which are very useful when you add the appropriate libraries on top.

Edit: the other big issue here is that blocking operations (e.g. anything in libmysql) are not automatically deferred to a thread pool -- that has to be done manually. Ruby's fibers and Python's greenlets are magical, but not magical enough that you don't occasionally get tackled by fantasy creatures materializing from the aether. If that makes sense.