The problem is that I see a lot of projects written in languages with single-threaded runtimes (python more often than node) that become difficult/expensive to scale and extend down the road. I loathe the idea of a rewrite, but sometimes the initial language choice lacked forethought to the point where it makes sense to rewrite in something that actually can make use of all of a machine's processing resources.

Things like greenlet and gevent (and likely napa.js) are band-aids over the underlying problem.