The goal of a talk-page is often to have a discussion, which entails replies and replies-to-replies, etc. I'm surprised the "Add this page to your watchlist" is not checked by default (I'm pretty sure it used to be prior to the new talk-page editor system), or even in a prominent location (odd to consider it an "Advanced feature" and in a collapsed panel).
Topic on Talk:Talk pages project/New topic/Flow
The default is exactly the same as with the old system (configurable in your preferences). And the watchlist checkbox is not prominent because topic subscriptions will be prominent instead once it’s out of beta. (I personally don’t believe in topic subscriptions, though, as while it avoids the clutter from topics not interesting me, it uses a heuristics, which is inherently up to 99% correct, and it may happen that I simply don’t get notified about a message because it happens to be in the other 1%… But that’s the officially promoted notification system on talk pages.)