I don't know how you can say both can be "destroyed" in a high pace game. I can't even remember the last time Barzagli has been beaten, let alone destroyed.
Chiellini does get way too much of the ball in attacking areas. It's the function of our system when Pirlo is marked. Him and Barza are the only men open but in any case, has nothing to do with his defending. Chiellini's defensive mistakes have mostly come from wide areas because he's playing as a wide CB so to speak.
The differences between playing CB in a 3 man and 4 man line are quite stark. I mean, in a 3 man line you're basically man-marking the whole match which would be hell for any CB. Chiellini hasn't been as poor as you think IMO. The only CB with adequate cover is Bonucci. Barzagli and Chiellini have no real cover, they're basically man-marking.
And I'm not trying to say they're all world class, that's just you putting words in my mouth quite frankly. Barzagli is definitely world class though.
But honestly, how many CB rotations in a 4 man line would you take than ours? Surely not more than 5.