Weren't there rumours earlier of a cash+Mangala offer from us?
The only way I can see us in for Otamendi is if Mangala leaves, and I think that it would be premature to call it a day on him after just one season where he was rapidly improving towards the end.
Denayer was obviously promised a decent path to the first team when he signed the new contract not even a fortnight ago, Demichelis is old enough to be the veteran back up who doesn't mind sitting out.
Otamendi-Kompany as first choice, Denayer as third choice, likely to get a good amount of game time when we rotate and also when VK inevitably gets a month long injury. Demichelis is the reliable 4th choice, ready to step in if there's another injury or Denayer isn't good enough. That's a balanced group of 4 CB's in my book.
We'd also only be spending a small amount of cash on top of the trade, so it makes more sense financially when we are already looking at a summer with £180m+ spent and only £30m brought in so far.