> JS event loop is preemptive

I think the word you want is "eager". Preemptive usually refers to things like signal handlers: when the signal is received the handler "preempts" normal execution without waiting for an explicit yield point.

In any case, with the WebLoop, Python coroutines stay lazy. The fundamental primitive a Python event loop needs to implement is call_later() which maps fairly cleanly to `setTimeout()`.

The reason we want to use the JS event loop is that the JS event loop is where all the actual I/O events in a JavaScript runtime happen. If you create a second event loop and run it, it will block actual IO on the JS event loop. So making uvloop work would be pointless.