With 2:00 left and the Colts with only one timeout, a successful conversion wins the game for all practical purposes. A 4th and 2 conversion would be successful 60% of the time. Historically, in a situation with 2:00 left and needing a TD to either win or tie, teams get the TD 53% of the time from that field position. The total WP for the 4th down conversion attempt would therefore be:
(0.60 * 1) + (0.40 * (1-0.53)) = 0.79 WP
A punt from the 28 typically nets 38 yards, starting the Colts at their own 34. Teams historically get the TD 30% of the time in that situation. So the punt gives the Pats about a 0.70 WP.
Surely, Belichick would not have been criticized if he had punted and the Colts had driven down the field, but based on historical stats, he made the right choice. That would have been the conventional approach, and would not have been controversial.
P.S. Matt Millen said "well, those stats don't take into account the fact that the Colts have a better-than-average quarterback." True, but they also don't take into account that the Pats do too, not to mention the fact that, if having Peyton Manning increases the probability of a Colts touchdown on a 29 yard field, it also increases the probability of a Colts touchdown on a 70 yard field. To me, that's close to a wash.