Forstall had one great contribution.
He convinced Steve to do "full" OS X on iPhone.
Beyond that, nothing. The OS tgat resulted was the work of many people not named Forstall.
Beyond that, he was a something of a jerk, a self-serving employee and had no real vision of his own.
The other good thing was that he was loyal to Jobs.
But post-Steve, he was very much a liability.
If he had some humility and played well with others, he might be an asset. But he's not. And...