Okay, hear me out here. Dominion was made by Certain Affinity and only works on select maps.
Extraction was made by 343 and is available on all maps.
Certain Affinity did 2 map packs, 343 did one.
My guess is that initially, all 10 initial maps (save for the Forge ones) were meant to be made by 343, and all 3 DLC maps by Certain Affinity. For whatever reason, however, this changed.
By extension, gametype availability had to change. I believe that Dominion was meant to be the "DLC gametype" much like Extraction; on-disc but no playlists, no commendations. Dominion is a way meatier gametype and it'd make sense for 343 to do that paywall bullshit.
My theory is that for whatever reason, Dominion and Extraction had to switch places, along with 3 on-disc maps. The question is, which? The most reasonable assumption would be the maps that can support Dominon, meaning that Exile, Vortex, and Longbow could all be potential options. It does make me wonder what 3 maps 343 are going to make, though.