[Paul Moore email@example.com]
I did consider what I would have done on Discourse, and came to the conclusion that I would have done exactly the same - I've no idea how Discourse would help with a "here's some things I thought of that I felt needed saying while reading this thread" post.
It wouldn't, and nobody would really care. It's when a technically off-topic sub-thread _grows_ that it becomes "a problem". Sometimes you just can't gauge interest in whether it will without making a start. If people pile on, the very lack of a fully-threaded view in Discourse is what _drives_ people to split it off to a new "category" of its own Which is a better outcome for everyone! If you do care about the new category, it has its own space wholly dedicated to it. If you don't care, don't follow it, and you'll never even know that it's still going on.
Obviously I could move the reply to a new topic, but I could just as easily have changed the subject in the mailing list.
But people don't. If this sub-thread keeps going on, someone eventually _will_ change the Subject line, and then you need "clever" software to show you that it's still the same sub-thread, and it keeps getting sent to everyone on the "python-committers" list whether they want it or not.
So without meaning to ignore your smiley, I don't think it's really a fault with mailing lists, just with how people discuss things ;-)
In the absence of trying it for yourself, you could, e.g., look for what the people who designed the system had in mind. The lack of a fully threaded view in Discourse was 100% intentional, not due to, e.g., laziness, or lack of time or skill.
Here's a start:
I'm not necessarily endorsing those views, but I did take the time to try to find out _why_ they did what they did. It wasn't capricious. There are things I do and don't like about Discourse, but _which_ things are still changing for me over time ;-)