This is not quite right. The userland dispatcher to run next thread is not a context switch: the TLB doesn't need to get flushed for instance. Furthermore many userland threads can be woken at once, and don't incur context switches when they suspend, prompting the next runnable thread to run.

If you're willing to put in a lot of compiler work userland thread switching is a function call, literally. I don't 1:1 threads with small stack are going to compete with that.