The thinking behind signing Demichelis was to add experience and sign a player that knew how to defend the way Pellegrini instructs the side. Though he took some time to settle in England he's been our most consistent defender over the past 18 months and has helped Mangala settle in the side during a period where Kompany has struggled with form and fitness.
Though the SPL isn't the pinnacle of football, we've sent a young player there and he's flourished and earned a call up to the senior Beligium squad where he's also proven his ability. So far City have looked to build for the future through investment in the academy, and also add a mix of experience and younger players who are ready to make the first team.
At centre back we have that combination. Demichelis, Kompany, Mangala and Denayer. We don't need to add anyone to that mix and we shouldn't look to shun Mangala or Denayer at this stage. This season can be the last with Demichelis' adding his experience and nurturing the others. I'd then either want a more experienced defender to come in and replace him, or to see Kompany step up to that mould.