To do that, I think they need to start splitting up their games more.. instead of trying to make a release, SP + MP. One part is always going to get more love or be hampered in some way by the other.
IMO, I'd like to see Gears and Halo just have their own separate and isolated MP suite releases.
That way when it comes time to do a new single player game they don't have to throw the baby out with the bathwater, it can be it's own thing, MP wouldn't need to start from scratch and change all the things people liked about the previous release for the sake of it being a new release. Then the single player can get the attention it needs and won't have a team split up trying to juxtapose how things will work from the single player to the multiplayer.. you could even have gameplay take a giant leap in another direction (ala God of War) and it wouldn't ruin the MP because it'd be it's own thing.
I'd love for that to happen. I think you're right; there are inevitable compromises when you try to do an outstanding SP and MP game at the same time. I think you'd get better quality if you separated them. And on a personal level, I don't like paying for content I don't use.
It's hard for me to imagine the industry moving in that direction, though. It would be a big shift in how things work. Plus, people might object to paying separately for both SP and MP (presumably $60 each), when now they get both components bundled together for a single price.
I like the idea, though.