We already have 2 of the top class ones you can't find in Italy any more, we just need a backup.
Last season we were using 2 CBs, so we needed 2 starters, one decent sub and a youngster.
But now we are using 3 and we are returning to Juve. We really need to add both in quality and quantity.
Caceres, Bonucci and Sorensen are great for depth and we need three great starters too, we have as you said, but we need one more starter, for the derbies and CL mostly, someone who doesnt commit Bonucci's mistakes, but being old enough to become a sub when Bonucci and Caceres come of age.
So we have enough depth, but we do need a quality to start to complete our defense.
So no more half measures, we need a top quality CB now, we have been dodging this problem for years, time to invest there!