What other options did Alonso have over the last 5 years? Red Bull? Unlikely. McLaren, not until the dust had settled on 07, Mercedes? Well until recently they were worse than Ferrari.
He was on a great wage, in a race winning car and racing for one of the biggest if not the biggest name in motorsport. Must have been a tough call. Half way through his first winless season he engineers a move to McLaren.
McLaren's performance levels have been dipping year on year and Honda have no track record as an owner or engine supplier of being quick out of the box.
There is nothing to suggest (yet) how the team will perform next year. Of course the presence of Alonso within the team will lift the results. He is a far far better driver than KMag and to be honest, Button, but will that be enough to see a top step this season? Who knows.
If Mercedes are the car to beat once again this year and IF Alonso has a peformance clause in his contract and IF Rosberg leaves Mercedes, what odds on Alonso staying at McLaren?
There are a huge number of "If's" between now and then.