I'm not sure I understand your point - it seems to me you are just validating what I have been saying. Humans have survived transitioning to a sedentary lifestyle - just as horses can transition to confinement. But at what cost? Diabetes, heart and arterial disease, and obesity are off the charts. Sure we live longer due to medical advances and a far higher birth survival rate, but we are soft, weak, and far less healthy a species than when we were living under more natural conditions...
Diabetes, heart and arterial disease, and obesity, are off the charts because of how we eat, combined with a sedentary lifestyle. That, and we now live long enough for our hearts to suffer.
If I fed my horses grain & sweets and didn't give them exercise, they would get fat too.
But until I was hurt 4 1/2 years ago (thanks, Mia), I was running 4-5 miles daily. It took about 45 min/day, and in return I could do a 6 minute mile at 50. I was able to start up running again in March, and while it is tougher to start at 55 than 15, I'm slowly getting back in shape. And that is all it takes to keep my heart in shape.
Horses, from everything I've seen, are even easier to keep in shape. If you feed them too much, leave them in a 12x30 stall and let the muck build up, you'll have a true "American" horse, with weight problems and heart problems. Hoof problems too. Keep the corrals clean, monitor their feed & ride them 4 times/week, and they'll do fine.
Heck, my DOGS are doing fine, and they aren't running 25 miles a day like a wolf would. This 'gotta be natural' stuff drives me up a wall. Nike has ruined my favorite running shoe because of a current fad for a 'minimal shoe' - ie, one that doesn't protect your feet and joints. So they cut back on the cushioning and support so some eco-turd can feel closer to nature, and my 55 year old knees were hurting in 2 weeks. When I'm running in the desert, the last thing I want is to feel like I'm barefoot...
The problem with your claim of needing pasture is the large number of fit, sane, relaxed, content horses without it. I don't know of ANY non-pasture living horses who have related behavioral problems or health problems if they are fed properly and ridden regularly. I don't think the problem is pasture/no pasture, but involved/uninvolved owners.