This probably belongs in the Software forum more than the books & movies forum, but oh well.
I was pretty surprised to hear that Bill Roper and the three other founding members of Blizzard left the company. In case you don't know, Vivendi Entertainment (the company that owns Blizzard, as well as other software companies like Sierra) wants to sell off its computer gaming division due to the fact that it's not as profitable as they'd like. This, of course, has led to numerous theories that Bill Roper and company resigned because they heard who was going to buy Blizzard and didn't like it. This is all just rumors and speculation, though.
(On a semi-related note, I hope that Electronic Arts dosen't buy Blizzard, or else every major patch that Blizzard normally offers for free will be part of an expansion pack instead. Just look at the way they've handled The Sims, that game has what? Five, six expansion packs?)
It does suck that those guys have left, especially since they played crucial roles in the development of both Diablo games.
But in the end, I don't think that you have a whole lot to worry about. People were saying that id Software was doomed (pun intended) after John Romero left to form his own game company, but that proved to be completely untrue... and John Romero is still trying to live down the complete and utter disaster that was Daikatana.
I believe that Blizzard will continue to publish high-quality, addictive games. Unless, of course, the majority of employees at Blizzard all decide to quit at once.