Left Juve because he wants to play on the right and on the front foot, apparently. And probably also because he wants to win titles and keep his EUR300K/w salary. I read earlier in the season that he was frustrated Upamecano was playing ahead of him. If only we got KMJ, we'd be sorted in the CB position. But alas we're a tier 2 side these days. He's certainly ambitious and hungry but whether he has the ability and fitness is another thing.
I really don't think the club should pay any player other than a genuine world class player more than 300K/w though. The likes of De Gea, Rashford, Sancho are not worthy of it and we should let such players leave as we used to under SAF.