What is with all this personal emotional feely stuff?

Yes, it's blatantly obvious that I didn't like Halo 4. I'm not keeping it secret that I think the game is rather lacking in quality in many areas.

That has absolutely nothing to do with why I'm saying that Halo is going to have to start fighting and delivering to maintain high sales on future rounds.

I'm saying that it's going to have to fight for it because player interest is low. A lot of people took issue with a lot of Halo 4's gameplay design choices in campaign, and the multiplayer had an unusually steep dropoff for a modern FPS of its calibre. People sometimes try to brush off the latter as somehow "normal" for the modern games market, but it's simply not; other not-quite-top-dog games like BF3 were able to hold their momentum extremely well over the same time period.
And historically, when an entry in a franchise sees this type of reception, reduced sales later on is not unheard of.