I'm not really surprised by the fact he's quitting. As I said on the very first episode the chemistry between Clarkson, May and Hammond that made the show so popular (even though they were just three middle aged blokes acting like kids) just wasn't there between him and Matt Le Blanc in particular. I have no personal animus for or against Evans but I think the BBC needs to take the blame for this. They had the opportunity to totally revamp and refresh Top Gear and take the show in a completely new direction instead of desperately trying to keep the old format alive with new people. The 'Star in a Rally Cross Car' was never going to work, the 'comedy' was forced and didn't sound at all natural and the road trips were basically badly done rehashes of what had already been done.
What they could (and likely should) have done is to take Top Gear all the way back to it's original format and make it a straight car show as it was before the Clarkson/May/Hammond era. Given Evans genuine love for and deep knowledge of cars it could have been exactly what the BBC's mandate is supposed to be, a show that would inform, educate and entertain.
If Top Gear is to survive at all they need to let go of things like the Stig and 'Star in A ..." and move the show onto totally new (or perhaps old) ground. The show worked with JC/JM and RH because the three of them genuinely enjoyed working together and knew each other very well and that showed and their enthusiasm carried over to the audience both in the studio and at home. It never had a chance of doing that with the current line-up and frankly they were rather cowardly to try and make the presenters do it.