Thanos destroyed THE Marvel Universe, and then brought it back. Not a Multiverse, or multiple Mutiverses.
People who are constantly throwing Starlin's words around, should at least be sure of what they are saying.
Interview with Starlin:
http://www.adelaidecomicsandbooks.com/starlin.htm
Along the way, Starlin has written some of the most controversial comic books of the last 25 years. Starting with his Death Of Captain Marvel, the first ever graphic novel published by Marvel, through to the death of the second Robin in the pages of Batman, Starlin has worn the mantle of the killer of characters. To this end, he took it to its (logical?) conclusion recently by slaughtering the entire Marvel Universe, only to have the bringer of death, Thanos, sacrifice himself to bring life back to the MU.
Here, Jim says “The rest of the Universe.” Nothing about “Multiverses.”
JS: Well, that was the challenge. To start off with is the fact that I did sort of leave him as either he was dead, or he’d just gone through something that no other character in the universe had ever done. And so I opted for the latter and when he was, basically he was God, it never said that he was actually going to go and die; he just said that was it. And, as a result, he put himself back together, along with the rest of the universe, but has gone through a bit of an epiphany while he was at it. And the series starts off finding Thanos deciding that it hasn’t worked being a conqueror and all that, so he’s going to give the lighter side of life a little bit of a shot. Try and be a good guy. But, of course, he’s Thanos, so, well you can just imagine how well that’s going to work. <laughter>