Rather than taking an existing topic in a radically different direction, use Reply as a Linked Topic.
That’s exactly what I did here. Wide-ranging conversations can be fun to have, but they’re hard to follow in retrospect and as a thread-less platform Discourse just isn’t very well equipped for long-running asides.
I withdraw my objection, I have to admit that I didn’t realize that reply as linked topic was an option. In my defense it’s a hidden feature, it hadn’t occurred to me that the arrow icon at the top of a reply was a button.
I’ll make a point of using it in future.
Edit to add: it might be nice to add a screenshot like the one above to the guidelines.
I agree it can sometimes be hard to discern in-the-moment when a new topic would be warranted — but it’s just generally helpful to try to keep responses focused.