I think the reign of EXO (though I like them) will heavily depend on when SM will debut SR15B. It's been an SM cycle that people end up stanning the new boy groups, especially big numbered ones, and they always seem to be the new big thing. Whenever this happens, other SM boy groups seem to take a back burner as the new boy group is promoted like crazy which could displease other fandoms who want to see their biases promoted as well. (see: just scroll through SM history and all the DBSK, Suju, SHINee stans complaining SM is not taking care of the mentioned groups enough)
It would be good if SM makes sure the EXO fandom is extremely stabilized before launching the new group because if not, this will happen:
NEW GROUP DEBUTS -> EXO STANS COMPLAIN ABOUT IT -> NEW GROUP GETS HATE -> NEW GROUP FANS DEFENDS NEW GROUP -> FANWAR ENSUES