U.S. Tanks are debatably the most durable in the sense that they all have nondoc smoke and good stabs, allowing for tactics that minimize damage taken.
Repair critical allows higher survival chance if fausted/mined; and the presence of REs and vehicle crews means USF tanks can get back into the fight faster than other all other factions except maybe OKW. (But USF has numbers.)
Decrewing USF tanks saves popcap and increased income, too.
So, figure that for all the bonuses above + the power of properly used and equipped Riflemen throughout the match, USF tanks don't require extensive durability because they have other strengths that represent the United States logistical and tactical superiority during the later stages of WWII, and also to encourage a unique micro-oriented playstyle.
I wouldn't bring those stats into it, the whole argument around blitz/panzer smoke/Twp/spotting scopes/crit repair is a bit too complicated to relate directly to unit survivability discussion. I'd say they all work out roughly equal
It's just a discussion about increasing investment to increase reward. Spend more resources on a tank - get a better tank, is this fair? I'd say yes if priced properly. Would it be giving USF an unfair upgrade advantage? I'd say not as Ost and okw get armoured skirts. Make the investment/utility of the upgrade give the player a reasonably balanced choice, and we'd probably see an even ratio of up armoured USF vehicles to skirted axis vehicles.
Most importantly does a problem exist at all, and does this fix it? I'd argue that in some games yes, the sherm's durability is a problem, but only the Sherman. Giving it the ability to take one more hit in the late game would be nice, and I'd be OK with paying a premium for it.