Are City that set really? Kompany has looked very flawed at times recently, Mangala looks very raw (and a club like City won't worry about the amount they spent on him if they decide he's not up to it), and Demihcelis is old and only meant to be backup... the fact he plays as much as he does says a lot.
Which leads to your final point, I guess. Otamendi may not be one of the greats, but there are very few centre-backs, even at the top PL clubs, who are. Now Rio & Vidic have gone, we're basically down to Terry, who hasn't got long. The likes of Cahill and Mertesacker are very good, but they're hardly greats.