You've got to blame BB for the loss because the play didn't work, but absolutely see the logic behind the decision.
Shutdown corner has an interesting take on the decision, and cites some statistics showing that, statistically speaking, BB's decision was the right one.
Shutdown corner: http://sports.yahoo.com/nfl/blog/shutdown_corner/post/Counterpoint-Pros-outweighed-the-cons-on-Belich;_ylt=AscUZAAQPaZA6kB1WQfCz5xDubYF?urn=nfl,202797
As I've stated before, I blame the loss on BB, but not because of the decision to go for it.
I don't buy it. What's the sample size on teams getting the ball with two minutes or less at the 28 yard line? The 53% conversion rate seems low to me, and it's pretty much irrelevant to an outstanding offense like Manning's getting the ball and needing to go 28 yards. I'd put the TD conversion rate at closer to 75%, and probably higher in those circumstances.
As long as Manning's TD rate is higher by a proportional amount on both the short and long drive, then the higher conversion rate makes Belichick's decision look even better.
Here is the math from the other site:
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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.
Statistically, the better decision would be to go for it, and by a good amount.
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Now, suppose that Manning is 1.5 times more likely to score a touchdown from anywhere on the field. That gives you 80% with a short field (even higher than your 75%), and 45% with the long field.
The probabilities then become:
Go for it win 68% of the time (0.60+0.40*[1-0.20])
Punt win 55% of the time
If you believe that Brady makes the 4th and 2 play more than 60% of the time, the case is even stronger.
And, suppose you believe that Manning is so good that he would win from anywhere. Then you have to go for it.