I wouldn’t say side topics are frowned upon, necessarily. However, they are just as disruptive as a split topic. I agree that discussions evolve. They also end. And new ones can be started. 
Personally, I find it annoying when I start a post and it gets hijacked with side discussion irrelevant to the original topic…and then I’m stuck getting a lot of extra notifications for the additional side-chat. I shouldn’t have to unsubscribe from my own threads. 