The same posters getting wildly excited about Koulibaly, are the same posters who were getting wildly excited about Maguire in the summer. There was a reason we didn't pursue our interest in Maguire and there's a reason we won't pay an equally outlandish fee for Koulibaly. If Napoli get desperate and drop the price significantly, then maybe it's worth a bit of a gamble.
Fernandinho isnt 6'4" and built like a 5 bed detached, in the same way long distance marathon runners aren't built like Anthony Joshua. The risk is far greater with injuries in part because of the size of him the older he gets. Although like @domalino has very rightly pointed out, Koulibaly has a very good injury record, so far.
There's a huge difference in building short term success before UEFA's cartel could build the FFP bridges and now. Remember when we last signed a 29 year old, no me either. Walker is the closest in recent years and he hasn't turned out too badly, so it's not a cut and dried debate but for the same reasons we seemingly pulled out of the Maguire pursuit, I think we'll do the same with Koulibaly. Unless De Laurentiis gets desperate which could happen considering current market conditions, let's hope so.
Koulibaly is 6ft 1