Florio over at PFT mentioned an article written by Geoff Mosher, with some points on why McNabbs career could be coming to an end, rather than a new chapter. They are intriguing points, but I don't think McNabb hangs it up yet. There are many teams that wish they had a QB that was ONLY as bad as McNabb was last year.
Donovan did have a rough year last year, but I still don't understand him getting yanked in favor of Grossman. Let's look at it for a second. McNabb was on a team with no running game, at all. His top two receivers were Santana Moss and Anthony Armstrong. The entire team was running a new offense. And not only was it a new offense for McNabb, but the progressions and reads were the exact opposite of the west coast offense that Andy Reed raised Donovan in.
I am not a McNabb fan boy, but if you spent over a decade doing something one way, for example your read progessions being from short to deep. Then you have to do it completely backwards, it will take an adjustment period. It was a little mentioned factor in the difficulty that McNabb faced in Kyle Shanahan's offense last year, that the reads would start with the deepest option first, then work the way down the tree to the check downs. This of course puts more pressure on your pass protection, and forces the QB to be much more patient. This doesn't sound too hard to adjust, but you have to think about the mental aspect of the game. If for over 10 years McNabb has a clock in his head that alarms at 3.5 seconds, and now he is in a system, where some of the reads are just progressing at that point, it is going to be a potential challenge to adjust, and will take time.
I don't know that McNabb is a top 10 QB anymore, but he also isn't one of the 10 worst starting QBs either. He will wind up somewhere and be a solid option. Don't be surprised for him to have another 2-3 rather productive years.