Regarding your suggestion of banning daemon threads: I happened to come across this bug not because of daemon threads but because of threads started by C++ code directly that call into Python APIs.  The solution I am planning to implement is to add an `atexit` handler to prevent this problem.

I do think it is reasonable to suggest that users should ensure daemon threads are exited cleanly via an atexit handler.  However, in some cases that may be challenging to implement, and there is also the issue of backward compatibility.