Thought as an Everton fan i'd give my two pennies here.
As good as he is, he's still young and learning the game and thats not what you need right now as you have a few of your own players like that. You need a top quality CB who is in their prime to improve you straight away - hence the links with Koulibaly.
As a player, when we signed him the Barnsley fans were saying he was more highly rated than Stones there so we were hopefull after Stones left that we had a replacement. Its took a couple of years and a loan out but this season he's been superb. He hasnt been as rash coming out, he's very vocal (hence the Everton fans captain shouts). He's not the tallest but manages in the air as he has a good jumping reach. He's brilliant on the ball for a CB (which is a must for Pep) and his timing of tackles is very good.
Plus, unlike Stones, Holgate actually relishes defending. Give it a year or two and if he keeps up his developmemt he will be one of the top CB's in Europe and I can see you coming back then.