And even without that, you had developer statements from as far back as October 2011 saying AC3 would end his story.
Developers also stated Ezio would get 1 game, then they added 2 games due to his popularity. Statements mean absolutely nothing and can be changed completely to suit fanbase and make more money
They literally showed 2 methods that Desmond could have survived, yet left a lifeless body behind (First Civilization had made a method to extract a person from their body and Subject 16 uploaded his brain in the Animus) within the game (and they didn't even properly explain what Desmond did or what other "save the world" attempts were there) and nobody actually checked and claimed his body as dead. Desmond just dropped to the floor and it ended (just like in Brotherhood). Desmond also should have gone crazy and died from the Bleeding Effect, yet he overcame that too, so just because they say something should kill him doesn't make it a fact until someone grabs his dead body and says he's dead and gone (which didn't happen)
In other words, the Developers intentionally made it so that they can bring him back if they want to, or leave him dead depending on the reactions. So until the next game has a different lead character and its flat-out stated he died IN-GAME its still possible for his return.
They can repeat the Revelations storyline and again have it focus on Desmond getting his body back by finishing off Conner's life. Then make AC4 about a new Assassin, or indeed make AC4 hundreds of years in the future with a decendent playing as Desmond to find out how he saved the world after reclaiming his body.