I personally find it quite confusing when a discussion is split/merged, or when messages are moved from one thread to another. This results in messages that lose context, and it is also possible that while moving a message into a new thread, other messages will lose meaning. So when I read a thread, I sometimes wonder where things came from, or whether they are in the right context. On top of that, for a regular non-admin user like myself, it is not even clear what are the capabilities of the platform in splitting/merging, and so I don’t know what to expect.
At the same time, I agree that conversations can take on topics that deserve separate threads.
What I suggest is that instead of “hard” reset by moving a message into a new thread or splitting, to split off an existing discussion, the relevant part of the existing discussion is quoted in a new thread. It probably also makes sense to add a reference to the original discussion where it originated.