I donīt know about your father or grandfather or their horses or where you get your information but it has been discussed before somewhere:
The military horse was in service until 20 years of age then mustered out and auktioned off to the civilian market, many bought up by the forestry dept. Here in Sweden by riding schools, these horses lived till long over 30yrs.
Colic and founder was not all that common that I know of and IR was not even heard of (IR is whatīs called a Welfare Sickness that can be due to poor breeding - over feeding/under worked or a combination of both).
There were good horsemen and there were poor horsemen;
Not to be mean:
But for someone to say that the horse knowlage was limited back when horses were a part of everyday life leaves me to wonder about that person own horse knowlage.
Such a statement too me sounds so juvenial:
"MOM AND DAD ARE SOOOOO OUT OF IT, THEY HAVENīT GOTTA CLUE.....MAN.....WHY CAINīT THEY STAY ON THE FARM!"
Every kid thinks they know more than mom and dad.....untill they have to fend for themselfs......then they wished theyīd listen a little more too what they said.
If past generations didnīt have some talent they would have starved and we would not be here.
Think about that
IR is not a breeding issue. Horses with no family history of it, at a good weight and sound can become IR. Diet can (and does) play a huge part. It's a bit like saying the high cholesterol is the result of human breeding and not diet.
As for old timers and what they knew. My grandfather knew that grain was (as he put it) hot feed. Corn was the hottest. He didn't know why. His father didn't know why, but they knew that it was and it needed to be avoided. Heaven help me if I was caught feeding our horses the diet you listed. I'd have gotten beat. Today I can graze year round, because I don't have the stock to feed that they did. There was a reason they had massive hay sheds at different fields and even barns just for hay, because they had to have feed for them and that meant having enough hay for them all (plus some excess just in case). While they didn't understand that there can also be issues with hay, they knew that grain was not good (and could be avoided), even if they didn't know why. (of course we didn't go on campaign with our horses, but who does anymore). But then they also knew that shoeing a horse was not good for it too. Now these were old timers. Born in the 1800's. For whatever reason they understood that just because something was a common practice did not make it a good thing. How they gained the knowledge I'll never know. At some point it was likely a case of seeing problems with some and not with others and sorting it out, but by the time my grandfather came along it was well know in the family. My grandfather was quite willing to say that the vet didn't know what he was talking about if it was something he knew to not be true (the vet kept insisting that my mare, with her white feet, needed to be shod or her feet would be ruined). In my case my mentors were knowledgeable, but I would not have called them enlightened since what they knew was not new to them. They would have said that a farmer should get more hay and not feed grain. They would have said "You don't have a clue about horses", but they'd have been glad to explain it to you. Their explanations were pretty simple. Grains = hot and hot is not good. No real knowledge as to why, but modern equine studies have given me a lot of respect for what they somehow knew. Of course I'm not saying I believed it all as a teenager (when I knew plenty of people who feed oats....even corn), but I knew better than to not follow their rules. In retrospect I'm happy that I did.
As for the survivability of former generations . They survived medieval medicine, but that didn't make it smart . Just means that if you were strong enough, lucky enough not to get sick, lucky enough not to get injured you a better chance of surviving. Today a man wounded in battle has better than a 95% chance of surviving if he reaches a hospital in time. 200 years ago most wounded men had a better chance if they didn't end up in a hospital . Horses surviving what humans do to them is a tribute to their ability to endure and survive "in spite of" what we do. It's hardly a measure of what are the right things to do.
Man didn't starve out because we'll turn just about anything into food . We domesticated animals to make it easier to do that. So long as they lived long enough to continue reproducing it didn't matter if their digestive system was working correctly. So long as it met human needs then it was all good. Seems some things haven't changed in 4,000 years