Is Prevent Defense Preventing Wins?

It’s a rather common cliche in the NFL, that prevent defense prevents from winning a game. Is it a believable theory? Certainly. And if you’re not sold, maybe you need look no farther than the Denver Broncos’ last three games. Granted, yes, they are 2-1 in those games, but if you are a Denver fan you know that those two wins should have been blowouts, not nail-biters (not that their 49-21 win over Philly was close, but at the start of the 4th quarter it was anyone’s game). In case you missed it, a 28-3 lead over the Patriots shrunk to eight points before a late defensive stand helped Denver preserve the win in week 6. The following game, the Broncos saw a 23-10 lead turn into a 24-23 Giants win in the closing seconds. And this past Sunday, a 28-0 advantage over the Eagles became 28-21 at the end of the 3rd quarter before the Broncos pulled away). Add to this the fact that three other teams have blown leads of 17 points of more in 2005, and a red flag should be raised. Surely, if you can build a lead that big, why can’t you hold it?
It would appear to shape up this way: teams will employ their defensive gameplan up to the point of building a huge lead, but once that lead is built, they will sit back and only rush four defensive lineman for fear of giving up a momentum-shifting big-yardage play. Worse yet for those teams, offenses can sense that and are not afraid to spread the defense out, or even take shots downfield. By switching to a prevent defense, a team is playing scared and straying away from what was successful for them early in the game. If a defense is wreaking havoc on an opposing quarterback early on, it can rattle his confidence. A good defensive football team will continue to attack, but strangely there is a trend of teams giving quarterbacks time to develop a rhythm.
This is not to say that the prevent defense is a bad thing, but for many teams it is in fact sending a message that "we are playing not to lose rather than playing to win". But then again there are some teams for whom the prevent defense may work, teams that can consistently generate pressure with their front four, thus not having to blitz to pressure the quarterback. But that is a select list, with only teams such as Tampa Bay, Indianapolis, and Carolina having the personnel to do so. So for the rest of the league, the message ought to be clear: play to win the game.