In my personal opinion: 3 years is a nice sweet spot for having development time for the game to come together, and for the previous game to realize it's full potential. In matchmaking, the progression often goes
When you have a franchise that's been around as long as Halo and does not need to reinvent the wheel between releases, 3 year dev cycles feel too long. With 3 year cycles people expect big changes, so when we don't see them people are left questioning why the need for such long development cycles when other devs are releasing their franchise games on shorter cycles and packed with more content. This is the underlying issue Halo had to face the moment CoD came out with annualized releases, showing the staying power of a constantly updated and maintained franchise. This is how I see it:
Year 1: Everything you said + the reality that competition is FIERCE. We're usually debating populations in here, so tell me.. what population will be left in a period where so many games are competing for that multiplayer space? I don't want to be playing the same Halo game where the only updates I see are Matchmaking playlists to include gametypes that I don't even want to play.. You gave the example of the 1v1 playlist in Halo 2, but that's exactly the problem with what you're saying IMO. You're forcing whatever remaining players you have to play in gametypes you want them to play, taking a gamble.
For me, a single year is a long time and the life of a single multiplayer game is decreasing these days unless you develop them in mind as a service (ie: PvZ Garden Warfare). What you're saying would be great if they're considering it as a service to be expanded throughout several years, but if they're going to repeat what we've seen the past two generations then I believe 3 years is too long for a franchise that's been around for over 10 years.
Year 2: As the sustain team spends less time bugfixing existing playlists, new things are tried. Historically, this is stuff where experimental playlists like H2 Challenge, 1v1, Maption Sack, etc come into being
Why can't this be Year 1 and/or why can't we have a CGB already? An entire year to test out playlists that I may or may not be interested in? These are gambles that we've seen have gradually lost them fans. Look at how popular games that let you create whatever you want are.. This is the direction Halo multiplayer needs to continue in, while still maintaining those core fundamentals for "classic" gametypes.
Year 3: The best playlists are figured out at this point and the worst ones are usually deleted. The game is fairly stable at this point and really finds itself Predictable, but refined. This is the year that's right in the sweet spot of being stable, but also getting a bit stale to those that have been playing for a while, and starts to make people hungry for the next release. A lot of people's good memories are from Halo 2 in 2006-7 and Halo 3 in 2009-10.
Halo has been around for over 10 years, so this shouldn't be the case any longer. Why should it be by year 3 that the game finds itself predictable and gains more stability? What standards are you holding the devs and playerbase to? Also, getting a bit stale by year 3? Again, Halo's been around for ages so I think a new game will get stale much sooner than by the end of its third year. I mean, look how hungry people are now for a new Halo and Halo 4 didn't even come out two years ago.. Sure we're in a new generation of hardware, but that's precisely my point; times are different these days. You have to consider the surrounding demographic of players and competing games if you want a beloved franchise to remain dormant without a new release for 3 years.
The two year intentional cycle harmed both Reach and 4:
I can't blame 343 because of their position with the studio, so these are special cases of forming a new team while Bungie was moving away from Halo. These years represented the transition period of this franchise, so I don't think they serve as a good indication of what you're saying.
Ideally, I'd like for the multiplayer to be a separate entity and to be considered as a service, but even that would be a gamble because what if it was based in Halo 3 settings? What options do I have if I don't like the base gameplay for the next game? Historically, these things haven't drastically changed within the life of a single Halo game, so all I would have left is either to leave the franchise or wait 3 long years? I don't think the people in power would want their fanbase to even think about those two options.