Actually I think you pretty much nailed it on this part. The biggest reason the Pats have been so successful this year in the passing game is that fact that NO team has yet figured out a way to get pressure on Brady. As you stated, defenses can't afford to blitz extra guys, as that will leave coverage breakdown somewhere downfield that Brady can exploit. The key to disrupting NE's offense is to try to get pressure on Brady blitzing 4. If they bring a 5th or 6th guy, they better be damn fast or the end result will likely be bad. There's few teams in the NFL today that are capable to doing this though. It's gonna take a D with strong/fast/creative blitz schemes to rattle Brady. Otherwise, he'll continue to do what he's doing. Sitting in the pocket and taking his good ol' time to find one of his receivers, who due to their superior skill levels, will eventually get open.
Yep, if I had to pick one area as THE KEY, it's this one. Can the Colts pressure Brady with their front four only?
If they can, Brady will not have time to look down the field and will have to resort to the short throw. I know Pats fans don't believe it, but the Colts secondary will limit the YAC to where the Pats won't be able to move the ball consistently just throwing underneath.
If they can't get pressure, I think they will be forced to blitz (something they rarely do). The Pats may get 40 if that happened.