I haven't switched away from Python (yet). But I agree that Go does look very compelling, and is basically the only performant and mainstream implementation of green threads.
I started the switch last summer. Haven't looked back since. Python gives the veneer of simplicity (ie the Flask hello world script, 5 lines of code approx?). On top of that the 5 different ways of doing x, or just wanting concurrency. Quickly Python becomes verbose. My current webapp is one executable that uses the standard net/http server and the autocert package for Letsencrypt.
Just for discussion would Erlang/Elixir not count as another performant implementation of green threads or would you not consider them mainstream enough?
Erlang has been scaling massively in the telecommunications industry for a very long time now even if it's not as popular as other languages.