Just a short "what if" question.
What if Snape survives?
How could he proof that he was working on Dumbledores order.
Except Harry no one would believe him.
And whatever has happened through 7 books, no one from the "official side" of the whizzarding world has ever believed Harry.
So, Snape would have been in Azkaban quicker than he could say "Dumbledore".