Well, most of the hugely successful multiplayer games have only a few modes people really play. CS:GO, Battlefield, LoL, Dota 2, WoT, TF2, Hearthstone and so on...(Overwatch will be the next one)
OK, many of them are F2P but that doesn't change that millions over millions play these games over years without getting bored. If you really like Halo multiplayer then you will also have hundreds of hours fun with "only a few multiplayer modes" (which is still more than most other games have).
If you need many modes because you get bored of every single one after a few hours then maybe the game itself is not that interesting for you.
That's my opinion about multiplayer games and it seems to be the case for most people. Normally even if a game has many modes you only play one or two of them (after you tried the other ones a few hours).
BTB is definitely missing because it is a mode which many played regularly without even touching other modes but at least we know that people who really understand how a map should be work on them till next year.