If we were talking about a lesser driver, such as Button, Heikki, Heidfeld, etc, then I might agree with you. However, Alonso has proved that he can drive any car and be quick in it.
2005 - wins world title with Renault.
2006 - wins world title with Renault.
2007 - moves to McLaren, in a car that is alien to him and finishes a single point from the title. Had he not been demoted in Hungary, he probably wouldve won the title. Important to note that without Alonso, the title winning team - Renault - are no longer competing for race wins or podiums. Its as if they are broken, in some way.
2008 - moves back to Renault and while initially the Renault is crap, as the season progresses, the car and Alonso develop to make the car, once again, a race winning car. Note that the team mate of Alonso, is performing poorly in comparison. Without Alonso, there would be no race wins for Renault.
The above shows that when Alonso leaves a team, it struggles. When he returns, as if by magic, the team begins to perform well again.
Ever since Alonso won the title in 2006, where he beat MS/Ferrari fair and square, I believed that he was the "daddy" of F1. Its possible that MS also believed this, which ultimately led him to retire. Based on his 2007 and 2008 performances, I believe he is still the "daddy" of F1 and that he will not have a problem adapting to a new team/car and winning, just as he did when he moved to McLaren and then back to Renault.