things to consider:
Most Halo 1 hardcore fans own Halo PC, and will continue to play that for the forseeable future. MCC is a downgrade for them: Less maps, they can already run the game at 60fps, and they can run it at much, much higher resolution with more effects. The community plays almost entirely via dedis already too.
Halo 2 is playable via LAN tunnels for Xbox players, and via Live for Live players, but soon won't be (?) because Microsoft shut down or is shutting down GFWL. The only reason you can't easily play Halo 2 online right now is because Microsoft shut down the online for it.
Halo 3 fans can currently play Halo 3 still.
Halo 4 fans can currently play Halo 4 still.
Going off these influences, Halo 2 will surely have the most population off launch because it's the thing people don't have right now (or won't have by the time it comes out).
Halo 3 will eventually take over as Halo 2 becomes infested with glitches and out-of-bounders and Halo 3 360 owners upgrade to the One. Halo 1 will have a minority due to being Halo 1 and mostly being played by console owners that refused to get the PC version. Halo 4 is still fresh in everyone's minds on the 360 and cheaper to boot. If you enjoy it now, you have no real reason to upgrade.
The real interesting thing I'm interested in is if they're going to try to upsell Halo 3 owners to Halo 3 One via Halo 3's MOTD, if they're going to update Halo 3 360's playlists finally, and what's in store for Halo 4 360's sustain at this point. Will Halo 4 updates now be done in parallel, or will Halo 4 360 continue to receive new updates while Halo 4 One players are playing some sort of weird "Best of Halo 4" version of Halo 4 matchmaking?