He was good for a couple, not world class and he didn't keep up that standard for that long. He played in a comfortable situation where he did little defending. In matter of fact during that 'world class period' he (or the defense) was frequently bailed out by Neuer.
Like I said, when your team only has to defend for 20-25% of a match there isn't as big of a pressure on you as a CB. It's not comparable to being in a team that has 50-60% of possession.
City shipped him off, no one wanted him when Bayern actively tried to sell him. Bit surprising for a supposedly "world class" defender don't you think.
Although Schweinsteiger got the plaudits, Boateng was probably motm for Germany in that World Cup final.
He was also incredible for Bayern during his peak. You won’t find a knowledgeable Bayern fan who will tell you that Boateng’s peak wasn’t outstanding. You’d also be hard pressed to find a Germany fan who’d tell you that either. Boateng’s peak was great. He was head of the defense for one of Bayern’s most successful sides ever.