The standard hog has been leaned up considerably, based on the idea that less fat is healthier (also because lard is less valued, but still).
Edit: And consumers can choose healthier animal sources without triggering any breading, replacing beef with chicken is widely considered a healthy thing, fish even more than chicken.
Sure, we now care about that. But we've been breeding animals for over ten thousand years. During that process, what did we lose? What of that can we get back with scientific supported breeding?
Or to state it differently, was the prehistoric cow significantly healthier for us than our current cow (or chicken)? And will it be possible to breed that extra "healthiness" back in?
As a biologist, my intuition is that there's been little change in the nutrient value of meat. Plants synthesise a very broad array of compounds which affect their flavour and nutrient value. Animal biochemistry is nowhere near as complicated, so there's not so much that can change.
There may well have been a shift in the protein/fat ratio of meat, but I guess that would be dwarfed by changes in what we eat and how we prepare it.
Edit: And consumers can choose healthier animal sources without triggering any breading, replacing beef with chicken is widely considered a healthy thing, fish even more than chicken.