> if you change run_forever with run_until_complete, then behavior changes: success_coro(5) will not be executed

Oh, that's a red herring. The reason is that the event loop stops when you use run_complete(), but the execution of success_coro(5) is still pending, and when you resume the loop (e.g. call run_until_complete() on the same loop with a different task) it will run.

So the issue is really the semantics of gather(), and I agree with Yury that we designed it intentionally that way, and if you want different behavior we'll have to provide a different API.