I would say most people don't care/know the difference between Infinity Ward/Treyarch.
That said, I don't think we will ever know exactly how MW2 would fare of Treyarch made it. If Treyarch releases MW3 this year, you could have shooter fatigue and less people buying. The last game was essentially CoD4 in WW2, and it was popular enough, but again it was going back to WW2 and might not have had as much appeal as modern war. I think Activision will push out barely changed sequels until they stop selling, then find another franchise to exploit. See: Tony Hawk, Spider-Man, Guitar Hero, etc.
With any luck, they give up near the end and give it to a decent smaller dev who makes the first good CoD game in years, only it was really rough despite the right ideas. But then they will destroy that studio. I want Web of Shadow 2 :'(