The general masses of gamers, I'll call them the mainstream gamers, they have moved on from the Halo franchise, probably for good. Halo 5 will probably be purchased by them because they're going to want to try/play the latest Halo, but this fall/winter, there are going to be too many shooters to choose from. Halo used to be the heavyweight contender, now it's a featherweight.
I have a passion for Halo, and once in a while, I really get the urge to fire up Halo CE because I want that Halo fix...but after one or two attempts, I just turn it off and go hop on my PC or play something else on the Xbox. The people who stick around MCC have managed to ignore the issues...they're the true diehards.
You make a good point, if the game is exciting to play and works from day one...people will stay...but if 343 is launching a game and saying..."stay with us for at least six months till we fix this game"...people are gonna laugh and say goodbye.
Star Wars has great branding behind it...it puts Halo to shame...but DICE is developing it...and if that game doesn't work day one...then we've got Battlefield 3 and 4 all over again. Battlefield 4's launch was a mess...not as bad as MCC, but it was still a mess and took in my opinion, six+ months to get it fixed.
Furthermore, specifically console gamers, they want to pop a game in, party up with friends, find matches, play competitively ranked with their own friends on their own team...that's Xbox Live gaming right there...and when a game doesn't allow for that to happen fluidly, kiss any potential community goodbye.