Caceres is 26 and Ogbonna 25, that gives us good scope for the future.
Fast forward 3 years and we are left with:
Barzagli 35
Chiellini 32
Bonucci 29
Caceres 28
Ogbonna 27
In a 3-5-2 scenario: Bonucci/Ogbonna duel will continue and develop. Caceres in theory would be the first choice DRC with Barzagli filling in the 'experienced veteran' role Ferrara used to, much depends on Barzagli's fitness. Chiellini I'd imagine would be more of a stay at home DLC in the Montero role by that age. Ogbonna could also play that role as a more advancing DLC, as could Caceres.
It would leave us with probably bringing in a defender through the ranks to become the 6th option and eventual Barzagli replacement. By which time Masi would be 24, Rugani 22, etc.
I'd say we are good for 3 years without massive investment back there, but it all depends who emerges or becomes available.