I don't know why people keep saying this. It's possible but there's no reason to think that.

Not communicating is completely consistent with Satoshi's behavior - he was very protective of his identity, so he would have to stop communicating eventually if he wanted it to remain secret.

I strongly believe we'll never see another message signed by him (either a transaction with his early mined coins, or a pgp signed message).  But that's most likely because he doesn't want to, not because he's dead.  I'd say odds that he's stayed alive since 2011 are the same as anyone else his age.  Let's say he's 50.  Based on life expectancy stats, his odds of dying in the last 3 years is about 2%.