Just because they're leaving Halo doesn't mean they're on hiatus, they're putting all their time into the new IP. This is no different from other game companies, they can't be expected to roll out the same game year after year, they eventually need a new IP. Halo had it's run but now it's time to move on, Bungie as a company won't lose any ground. Activision seem to believe in them, and Activision are pretty good at being in it for the money.