As you say, this is a cut-throat business. No room for sentiment, if you don't perform you're out. So he must be doing something right ...
Can you tell me what it is exactly that he is doing right, because for the life of me, I cant see it?
...and he's one of the fittest drivers on the grid.
Fitness counts for very little I'm afraid. MS brought the whole super fitness aspect to F1 driving, however, he did this when he was exploring other means of improving his performance and felt he had gone as far as he could in other areas of improvement. He left no stone unturned.
Unfortunately now, rather too much is made of the required fitness level.
Massa isnt too fit, but he did rather well in 2008.
Similarly, when Mansell drove well in 1992/3, he did pretty well and wasnt particularly fit.
Fitness is important, but it wont make a great driver.
He can't still be trading on the reputation that he gained in 2004 at BAR. F1 doesn't work like that. Yes, Rubens out-performed him this season. But he's also a lot older than Button and won't be driving in F1 a whole lot longer if his contract was renewed. Sure, they could take a chance on another rookie to replace Button - but they know Button, they obviously get on with him (otherwise he'd have left in a sulk and joined Renault, as I believe tradition holds when you fall out with a team), so why take the chance?
Honda could've taken on Bruno Senna and Barrichello. This means you have a combination of experience and youth. The hope being that they give Senna 1 year to see how he gets along. Meanwhile they know that they will have at least 1 driver (Barrichello), who will be making the most of the car they provide.
With Button, he is going nowhere fast. We have seen this pattern emerge over the last 2 years. This is fact, based on 2007/8. There has been no improvement and I would argue he has actually gone backwards over the last 3 years.
If they kept Barrichello, the reason would've been based on past performance. They have kept Button based on the fact that Button doesnt sulk?