That's the opinion of a lot of people who take a couple of jokes and believe them. McNabb has worn down three times in his entire career, all for a specific reason:
2003 game at JAX
2005 Super Bowl game at JAX
2006 game at TB
See the pattern? Each game was in Florida. Something about Florida bugs him.
So, the wearing down part of your theory is out the window.
The main problem with the Eagles this season has been BEING UNPREPARED , STUPID PENALTIES , and SLOW STARTS . How are those things the fault of McNabb specifically? It's the coaches first, secondly the players as a whole.
I can't stand how people who fall for some badly spread theories on the Eagles. McNabb is actually one of the games' best quarterbacks. He has been slowed by injuries, true. The defense was good for years. True. But he's also a great QB. McNabb and Brady are two of a very small list of successful QBs with WRs who absolutely sucked. Brady has been a little more successful. True. But he's the best in the game. Manning is also up there, though with great targets. McNabb is in that top core.
Besides, your theory itself doesn't make sense in the first place. If he always was a bad QB who wears down in big games, the decline CANNOT be attributed to him anyway, it would be attributed to what WAS successful, which you say is the defense. Even though it's not true, it is notable that it contradicts your whole point.
Overall, McNabb is a very good quarterback. He has declined a little bit due to injuries over the years. But he was just recently an MVP canidate! The McNabb hating has never made sense to me. He's such a great QB, a good person, yet people are quick to jump on him like they are a T.O. or an A-Rod, someone with dislikable qualities. It's misplaced entirely.