Do you think they would retain more players and grow without MT's or with?
Do you think it's a natural decline in interest for these games? Or is it something else?
Is it because of how Multiplayer in general has been treated this gen?
There are a tons of potential reasons why certain Xbox IPs aren't as big as they used to be, and quite frankly, MTs aren't a very likely candidate considering the implementations in the biggest games available today. Halo before effectively stood alone in its genre, whereas now it has to combat a whole bunch of highly proficient FPS offerings, each with their own ways of retaining their playerbases. There's COD, Battlefield, Destiny, Overwatch, SW Battlefront, The Division and Rainbow Six all splitting the FPS pie, and all of them have the advantage of being available on multiple platforms, whilst Halo is limited to a console userbase that's roughly half of its direct competitor.
However, Halo 5 has been stated to have made more revenue than past games did via their DLC, and has a player retention that's supposedly better than any post Halo 3. It has a smaller audience than it used to have, but it's keeping that audience engaged, and the MTs have helped this, by ensuring that content updates are delivered to everyone that plays.
Forza Horizon 3 is currently consistently one of the top played games on XBL, generally hovering around Black Ops III (was one place above yesterday, is one place below today).
In fact, in the PAL charts we get each week, you can see that Halo, Gears and Forza all consistently sit within the Top 40 long after their releases, whilst even title like Uncharted 4 have long since disappeared from the list. As I said before, these charts are basically dominated by games design to keep their playerbases engaged long past the credits screen, and I don't feel that removing these aspects would benefit these games (much as it clearly didn't for stuff like Sunset Overdrive or Quantum Break), and would just cause them to die out faster.