Thank you for sharing this--its information that I have really never seen shared, and I think it is super important!
I've been working on an online chicken database, and I've been sobered by the information I've gleaned from it. Though many breeds were originally "created" to be more robust animals, the dedication to the breed's "standard of perfection" has, in my opinion, resulted in folks breeding creatures for their looks and singular defining trait, rather than their overall health. You may have an award-winning hen with all the perfect proportions and that special comb, but she'd never last a week free-ranging like a normal animal. Sigh.
More personally, we have been finding this to be true with some of our first animals too (we're still in the rather heartbreaking first-animals stage). We anticipated that they might be weaker from all the research we'd been doing, but it has been sobering to see just how weak some of them ended up being. We've been recently opting for any heritage or mixed breed animal we can get--mutts always seem to do better with their mixed genetics. Our endgame is to end up with our own, homestead-specific "breed" of animals that have been raised here and are used to the sort of life that can be had on our specific land.