Maybe unpopular opinion... Should Halo invert its focus? Currently it's multiplayer first, singeplayer second. If the multiplayer modes cannot maintain a playerbase then its not going to be a main driver of success. The battle royale and hero shooter crazes haven't left much room for the Halo multiplayer format to succeed these days: most of the potential players are focusing on something else.
I think if they could deliver kickass campaigns consistently that they could keep Halo as a successful franchise. If they keep chasing multiplayer it'll fade into obscurity soon enough.