I mean, I’m sure the reason part of the reason “we never used to have problems like this” is people didn’t know much about how to spot many issues. And if the horse wasn’t rideable, it got put down. There just wasn’t that much knowledge or advanced diagnostics and so forth. Additionally, if only the hardiest horses who could endure with relatively hard riding and hard saddles survived, there might be a chance they could also make do with suboptimal diet, pasture, and so forth. And perhaps more fragile horses were less apt to be used for breeding because of all this. I’m obviously not advocating this was better.
I do think there might be something to the idea that horses may have been ridden somewhat differently in the past–I was lucky enough to do an eventing camp at the now-closed Vershire Riding School in Vermont and in many ways the school was a bit like a relic in time–big pastures with many horses in them, immaculate stalls (that the horses were almost never in), horses being ridden all day, up and down hills, very simple tack, and many of the horses were quite old. But they also had long periods of downtime when camp wasn’t being held.
Horses definitely jumped bigger (and so did riders, sooner in their careers) in the past but there may have been more hacking out, and less hard technical demands on them. It also seems like people are less interested in owning sort of “square hardy boxes of do-a-little-bit-of-everything” horses, just like kids are specializing in sports more and more, earlier and earlier.