As somebody who thinks Halo 4 is the quantifiable narrative peak of the series with a complex and moving story, Halo 5 will always be one of the most disappointing follow ups I've ever known.
Halo is like Assassin's Creed in a way: the massive fan base has stayed with the series for different reasons. Some are in it for story and the lore and others are in it for the competitive online multiplayer. Some were even in it for the couch coop. So who do you make the game for? You don't have the time or budget to invest in everything. The list of demands has gotten too long for your business model. You will inevitably disappoint somebody. And by somebody, I mean a sizable portion of your most loyal, but divided, fans.
I mean, Halo is the only series that can be shamed and lambasted for the state of its multiplayer on the MCC, but have that same "broken" collection win the IGN user award for best remaster. The divide is as wide as the modern day drama/historical fiction fans in Assassin's Creed fans, and just as difficult to reconcile.
Who knows if there will ever be another Halo with a story as as 4 and multiplayer as good as 2/3. Halo might not be able to sustain the empire it built anymore.
There is, it's called Halo 5.
Halo 3 may have been good "at the time" but it's aged horribly. The MCC made that more apparent.
I never played Halo for the multiplayer, so I can really only echo the opinions of others I've heard. But you get the point I was making either way.