Well, Liverpool signed van dyk in January, same goes with city on laporte, Barcelona signed lenglet last season and probably were into market for very different areas in the team, Madrid's no 1 is still Ramos with Varane his partner while scouting militiao all this while. Juventus as a fellow rival were not in for him as well while did they did bring back their defensive legend from Milan and were stocking money for Ronaldo. That just leaves psg who were severely dithered by ffp regulations, so i don't see which top teams were in for central defenders in the market.
At the time Jose signed a new contract there was no communication on future list of players and areas which needed strengthening. From that moment on till end of the window not once i remember koulibaly being discussed all summer at least in reports while we were wasting time on targets which weren't interested to come or not worth the money while we were looking for a fair deal. Kouibaly signed a new contract at the end of transfer window practically strengthening napoli's bargaining position and hike the price according to precedent set by Southampton last January. These shambles contributed to us being thrown into Europa league this season along with many more poor decisions of buying matic and neglecting fabinho for fred.
All this time since January to the end of the window we were never in for koulibaly and never came to realize this is the going rate for top defenders now and take advantage of ongoing discussions of new contract between Napoli and koulibaly while we were the only team needing a top tier centre back who could play from the back? They failed to move on Rojo as well and wasted his full wages for a year and jones as well.
The board is culpable for the mess they created last season and were totally caught sleeping on koulibalys contract situation as well.