I think that from the beginning Shanahan didn't like McNabb in Washington. 

I think that Shanahan did not belive that McNabb could get the job done. 

I think that Shanahan is looking for other QB's at any cost. 

I think that Shanahan will throw McNabb under the bus to save himself. 

It has reached the point over the course of the season in Washington, that this relationship is beyond repair. These two guys are just never going to mesh. No matter what happens at the end of this season, it is just not going to work between these two guys. 

Ranging from the lack of trust, to the fact that Shanahan is on the hot seat in Washington, things just didn't work out. It's a shame to see a QB like McNabb not work out in a team that needs so much help, and it may go down as one of the most failed experiments in NFL history. 

The way this whole thing is going to shake out is, McNabb is going to leave at the end of the season. Even if Shanahan goes before, McNabb will still want to leave. His entire career in Washington has been tainted and there is just no way to recover from something like this. 

With all of the teams interested in McNabb, his career is far from over. It's just done in Washington.