When did you last see a CL winner or at least serious contender with a pair of 20-22year olds as starting CB's? Not saying it can't eventually happen given the quality of the two in question here but odds are sure against that notion, usually it's a good balance between experience and promising, and you don't need me to tell you that(see Real,Barca for ex with Pique-Puyol or Ramos-Varane), but the ideal scenario ever since last season would have been to have DeLigt/Demiral grooming next to Chiellini and not as much with Bonucci, circumstances unfortunately obviously didn't allow it, point is i don't agree, i prefer another year or two of transition and overall i'd feel much comfortable with a Chiellini - Deligt/Demiral partnership if we play with two CB's and ofc if Pirlo really considers 3 at the back then we obviously will be relying even more on the old guard, which wouldn't be that bad of an idea atm, provided Chiellini stays fit and Bonucci does at least the basics right ofc.
Varane had a bad game vs City, but has been a mainstay in their D for half a decade now. I strongly strongly strongly doubt that he would be benched for 20 year old De Ligt. At Bayern he'd have more of a chance to break into the starting XI, but most managers will opt for the experience of Alaba and Boateng over a 20 year old CB.