It's not in the cinematic, it's what Tyreal and everyone else says that tells the end of the world is coming.
Tyrael doesn't destroy the world... he destroys the worldstone. They don't say anything about the end of the world after you kill baal, they're talking about the end of the worldstone!
Tyreal says that if he left the worldstone, then Hell would be able to get onto Earth. He says that shattering it will save mankind but with some consequences. That's a plot hanger there

. Hope they do make a new one.
Well I think if Blizzard makes a sc2 (or a sc3 if you want to call it that) they will most likely try their best to make a Diablo 3. And they WILL make a sc2 (or 3), they say so themselves
here.
that would be so cool if they made a d3
I dont see why Blizzard couldn't simply assign a new team to a Diablo 3. its been done many times before; not always sucessful, but occasionally. It would certainly be a wise move, considering the sucess of past Diablo games, if the new team could make a half-decent game. But then again, Blizzard never makes "half-decent" game. Thats not Blizzard. If it's not polished to a blinding shine, we're not releasing it.
As long as it isn't 3-d'ed, I'll buy it.