I'll miss Jeff, but he's not the end all of why Overwatch is a fantastic game. Bear in mind "faces" are often just that... the "face" of a large group, who generally is comfortable being in front of the crowds and talking with everyone.
Hearthstone just wasn't the same without Ben Brode.
I wonder if the reason Jeff left was that he was asked to leave for taking so long with the game...? I would rather he not leave as he has been the face of OW for a long time and beloved by the fan base. But, this is a public company and deadlines and dollars matter...
I highly doubt it. Very visible employees that are community-loved are incredibly stupid to let go of. Jeff's community fame is worth dozens of other employees' worth to the company.
My guess is that Jeff wanted to move on, either due to issues internally, or for something different.
My money is on Jeff Kaplin joining mike morhaime's Dreamhaven. That studio seems to be stealing lots of top executive talent from Blizzard.