Very interesting, and the perspective of people who worked in the industry is always appreciated.
I think we have to be careful comparing contemporary and WW2 aircraft design, because for example, a modern fighter-bomber (or "strike fighter" is I guess what they would call them today) can drop a precision munition from 10,000 feet and still hit a target. In WW2, when fighters were used as CAS or for strikes, they had to get very low. Well within range of light AAA and even small arms. This was potentially very hazardous of course, even if you had an extra 20-30 mph of speed at that altitude.
Needless to say, flying at 30,000 ft there is relatively little threat from flak, and since Mustangs flying escort missions for strategic bombers, their most important role, mostly faced enemy fighters, they didn't get hit with return fire as often (except from some of the 'heavy fighters' they sometimes engaged with). Mustangs could also escape from most enemy fighters due to their very high speed at altitude, always a very important trait for fighters, which also enhanced survival rates.
In Theaters where they were used in a more tactical role, P-51 were noted by some Allied pilots for having vulnerable cooling systems, since this extended to the wings. By comparison the Typhoon, Tempest or the P-40 had most of it's cooling plumbing in the front of the aircraft which was a little bit harder to hit, especially by ground fire. Aircraft like the Thunderbolt, the Wildcat or the Hellcat, or the earlier Fw 190s or the La 5, had air cooled engines which didn't require radiators, though the P-47 had the turbo system which could be vulnerable.
As for whether elminating some of the hardening of the F-35, whatever that actually means in detail, I guess we will find out. Some of the design decisions for that aircraft seem to be a bit questionable, but we won't know until they are in action. In the past, the US has made decisions like removing guns from aircraft because they though missiles would always work, which didn't turn out to be true... hopefully the stealth tech on the F-35 will prove to be extremely effective if we ever have to use them in a real war. Hopefully later rather than sooner.