That's not true. The intention behind these games are to be played long term. Whether there's yearly iteration, or consumers are playing the games for years at a time - franchise fatigue can set in. It's not the number of games in itself that contributes to franchise fatigue, it's the fact that there's little to no meaningful advancement through out your experience with the franchise - which happens whether it's yearly iterations with minimal improvements or a game every three years with marginal improvements.
No, it's completely true. What you intend to do with a franchise and how viable it's mechanics are are separate. Of course developers want their games to be long term endeavors, again it's all about profits. Bleed the rock as long as possible, etc. The idea that all game mechanics lend themselves to that is foolish. You control that fatique by the frequency of release, but regardless of how often or how little you release a game. People will get tired of them, nothing stays on top forever. Videogames and other products are no different. That is exaclty why these companies have more than one franchise and why they invest in new ones. Because nothing lasts forever. Very rarely do you get any company that only has one product that remains unchanged (WD40). You're not going to find companies that put all their eggs in one basket. Even with companies like Facebook, they're constantly applying new things (and rolling them out incrementally) to maintain/increase revenue.
The bold isn't true at all. You can innovate as much as you want, but frequency of release does very much play a role in fatigue anyone who has released a product/software will attest to this. If you release too frequently people will get annoyed at constantly feeling like they have to purchase a new version and will feel that their previous purchase was pointless, eventually they'll stop purchasing altogether regardless of how much you "innovated" because consumers will simply look at it and say "meh, they're just going to release a new version in X amount of time anyway". That and if you're always radically changing your product in conjunction with frequent releases then you're scattering/fracturing your userbase and there is no legacy at that point.
Releasing in high frequency works best when you have a commanding control of that particular market (point in case, COD, Madden). Again neither Halo nor Diablo release in high frequency. Diablo 2 came out in 2000, Diablo 3 came out in 2012. Diablo 3's failures stemmed from simplifying the game too much and mechanically breaking the game by forcing players to rely on the auction house in order to progress. There was never a section of gamers who wanted to play a stocks and exchange based loot game. A pointless "innovation" to which Blizzard will tell you was an absolute huge mistake. Hence they removed it after seeing their userbase continuing to drop. Their incremental adjustments to Diablo 3 were in the direction of Diablo 2. They realized they didn't need radical innovations that no one was really asking for. In fact during the Diablo 3 panel at Blizzcon yesterday they stated just that (They were chasing something that wasn't there ignoring how badly it affected the game and how that in turn affected user rentention).
Halo releases with slightly greater frequency, but as I stated. Halo has been on an upward trend with each subsequent release, so your argument of fatigue setting in due to no changes/little changes doesn't apply to Halo at all. It's telling that Halo 2 was still the most played Xbox 1 game on Live well into Xbox 360's life, the playerbase at times eclipsed Halo 3's. That is valuable data, it tells you where your consumers are focused at. Despite many issues with Halo Reach, the userbase continued to play the game long after it's release. The same could not be said for Halo 4 despite selling $20m more in its first 24hr/week than Halo Reach.
There is no fatigue with Halo's formula that prompted the change to the COD formula, and Halo doesn't release with enough frequency to really justify it either. Blindly following trends because they're "in" is a sure fire way to ruin your product, especially when you have data present that highly suggests those who buy into your brand weren't receptive to them.