Another point of view:
He's too good for benchDepended who are other players and in what team. At United, when there are Rooney, Chicha, Welbeck and Kagawa for 2 spots he should find himself lucky if he is in the bench. In Aston Villa he would have been too good for bench.
we ruined his career
No, he was well payed, won 2 medals and played with some wonderful players. He played in the second best club in the world in those years. It wasn't that exactly Barca, Real, Bayern were interested for him. Only City made a bid for him.
shame for him how he spent his best years at a wrong club
It is not. See above.
he's better than Chicha and Welbeck
He isn't better than any of them. Chicha in 2010-2011 and Welbeck in 2011-2012 shown more than what has shown him in all these years.
he would had won us the title if he would have played more
That is entirely possible (most likely not) but impossible to prove one way or the other.
he won us the title in 2011
Everyone who played contributed to that. But he scored a lot of goals for us in half a season. His contribution should not be overestimated.