Westbrook is leaving for LA. Plus Westbrook isn't a player that is coached.
Westbrook can't opt out of his current deal until summer 2018 at which point he'll be eligible to sign the new "super-max" 5y, $220mm deal only with OKC. I don't think you give up the chance as a coach to be part of a team that has a top 5 player like Westbrook. Lots of NBA stars can't be coached, but it really doesn't matter as long as you can win with them. Some, like Lebron, make their teams a whole lot better. Some, like Cousins, don't.
Even if Westbrook leaves next year, Donovan has his pick of any college job that he wants next year. Maybe it's just me, but I don't see a compelling reason other than some notion of happiness. The Brad Stevens storyline actually makes more sense, not that it's likely.
Note that Saban was 9-7 and 6-10 in his 2 years in Miami. I don't think he leaves Miami either if he goes 10-6 his second year and makes the playoffs...