Like some others have stated same could be said about multiple franchises. Halo is honestly one of the few FPS I like and is the only one I play on a competitive level. I'll keep playing as long as it's a quality product. I have high hopes for Halo (xbox one) with Gh057 and bravo working closely with 343i.

Its not bad, just not reall good. Its like an direct to DVD sequel if we would speak in movie terms. It the worst God of War game by far. It did a lot of damage to the God of War Franchise and I am not sure if it can recover from it. 

One of the few games which contains the full package. Story, competitive multiplayer, co-op, and play, create, share.

To answer your question, when it stops being high quality. So not for a LONG time

Well to be fair there isn't one definition of a Halo fanbase.

You have the lore fans and you have the MP fans, which to be fair, are responsible for the bulk of Halo sales.

The new Halo's, no offense to anyone who likes them, are shit. I don't understand how it sells at all. You can see the decline of the series starting after Halo 3.

Halo: Combat Evolved- Great story, innovative gameplay.

Halo 2- Had great online multiplayer and single handily saved a console.

Halo 3- Introduced Forge, improved the multiplayer.

Halo 3:ODST- Introduced firefight, other than that it was lackluster and forgettable.

Halo Wars- Bland strategy game

Halo Reach: Boring and tedious multiplayer, pretty bland story, gameplay is the same, barely any innovations.

Halo 4- Multiplayer too similar to Call of Duty, Story is forgettable, gameplay is the same, barely any innovations.

In my eyes, Halo is already dying. It'll just take time for people to realize, but when it does, MS is going to be in huge trouble. Forza is already dying itself, if it's two big exclusives go, the Xbox One will pretty much just be a multiplat machine.

I stopped reading after "wars 2."

It's a competitive type shooter, as CoD is. Regardless of whether or not the general public plays it for fun, people love to compete with it. 

It's probably a big deal in US due to the huge advertising budget because here in EU people aren't particularly hyped for Halo.

E.g. Crash Bandicoot was far bigger in EU and 1+2+3+CTR+Bash+Wrath of Cortex+Nitro Kart has sold 1.80 mill on average (PS only) whereas Halo CE+2+3+ODST+Reach+CE Anniversary+4 has sold 1.51 mill on avg (XB only). Same thing goes for Fable 1+2+3 (0.76 mill on avg), GeoW 1+2+3+Judgment (1.29) and Forza 1+2+3+4+Horizon (1.12).

For comparison in EU (PS only): Metal Gear Solid 1+2+3+4 (1.74 mill on avg), Gran Turismo 1+2+Concept 2001 Tokyo+3+4+5P+5 (3.29) and FIFA 8-13 (2.39).

people are bored, but much like WoW, people don't want to think all the time they put on it was a waste. So people stick on it defending it like it was there own baby. Some let there kid starve to take care Of the game.

Its a mental illeness, you have to get cure. Its the only way. It is the farmville Syndrome.

For as long as the XB brand is successful.

I think thats your perception of it. It certainly didnt tell a new story and it was like you say sort of a filler story. But the gameplay and top notch presentations were there. It wasnt just more of the same, it refined the gameplay and presentation on the GoW games. I dont think the franchise is in such bad shape as you think, but the filler story does give it a less imperative status in the series. :)