I have asked myself the same question about wld. Forty five years ago I was still managing a stable that boarded forty to fifty horses at a time. Not one time was there anything I would consider wld. There was the occasional case of thrush which we treated with coppertox and that would be the end of it.
Don't recall any with symptoms of ir or cushings either.
We fed lots of grain, usually a gallon at a time and sticky sweet. No founder, no wld.

Now I don't dare even give my two grain, I, too, wonder what happened.

Glad I'm not the only one lol. But seriously. What is different? I can see the highly specialized grasses for yield and protein. Back then dairy cows weren't expected to produce that much milk, so pasture was what grew. And that was the pasture the horses were on, former cow pastures. Feed....the only processing was rolling/ cracking. And it was grain grown locally, again less yield.
The McDonald's syndrome in horses also? Doesn't sound too far fetched.....

About 22 years ago I had a mare that got Lymes disease. She then foundered quite bad. It was believed due to the stress of the illness. I had a farrier that specialized in "fixing feet" and worked hand in hand with my vet. The first vet that did the X-Ray said it was so bad that the only humane thing was to euthanize the mare. The second vet (the one I normally worked with) said that she believed with the help of that farrier that we could save the mare.

The farrier came out the next morning (THANK YOU TO HIM FOR BEING SO PROMPT) and he put on heart-bar shoes. The mare was still sore but she stood better and was willing to take some steps. She was kept on an anti-inflammatory drug for a short while and of course, diet was changed. Within a few short weeks she was walking less painful. Shes were changed every 5 weeks and as the foot grew out it was decided to just put a normal Keg shoe on her. She stayed sound for another 20 years and was able to she shown and ridden like any normal horse. It takes time but there are things that can be done. That amazing farrier saved my horses life. Sadly he passed away and I have yet to find another that is as great as he was. If you have a good farrier, count yourself very very lucky as they don't often get the credit they deserve. Without good feet, you don't have a horse.

I pray that your boy can recover well from this, it is a long road. It is heartbreaking to watch.