The issue with a potential move for Koulibaly is that he no longer wants to leave Napoli and will not actively push for a move.
That leaves City in a very hard position to negotiate with De Laurentis who will demand a fee in region of Maguire/Van Djik.
The situation was very different six months ago when the the dressing room was in mutiny and most of the senior players wanted to leave.
Massive credit to Gattusso as he was able to turn it around and convince most of the characters to back him (Mertens, Insigne, Fabian Ruiz and Koulibaly).