Both can be destroyed in a high pace game.
Chiellini had good moments in 3-5-2, but his form fell too much and now he's rather average most of the time. Nothing indicates he'd be better in 4 man defense really. I don't think he gets the ball way too much, it's usually Bonucci who holds the ball. Chiellini makes way too many mistakes even without the ball, opponents get past him much more easier nowadays.
One of the best CB rotations? You make it sound like all three of them are world class players, which they are not.
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.