Yes, topics often develop in a direction which takes them some distance away from their starting point. This, as Mr. Mack and others have observed, is consistent with the the ebb and flow of real-life conversations.
It would be far from a good idea to lock topics for such reasons. Threads do get locked on here, generally if, as occasionally happens, they degenerate into a playgroundesque brawl, but such a move is always contentious and is one which we generally avoid.
I trust that these remarks have remained on topic.