Keith_Beef
Novice equestrian, accomplished equichetrian
And how long did those horses last as useful, working animals? A riding school, for example, that treated their horses like horses were treated in e.g. WWI would be quickly shut down!
When I wrote about those wars (Napoleonic, Boer, WWI, WWII), I was thinking of lighter cavalry for carrying out reconnaissance or skirmishing and striking at a long distance from the main body of the army. These units are more lightly armed, usually not armoured, and might have to carry rations and at least bedrolls (rather than requiring local populations to house them overnight).
If you think in terms of repeat business, having one horse that can carry, say 18st, isn't really enough for a riding school to advertise a weight limit of 18st, because what if that one horse goes lame and you have to turn them away? You also don't really want a horse working right at its weight limit all the time. So, we had a stated weight limit online, but we were flexible with it in reality- and we still had people we needed to turn away due to weight.
The problem of keeping enough horses to carry different ranges of ability and weight is a tough one to solve, you're absolutely right. Where I ride, there are close to 30 riding school horses, of which I think five or six are suitable for me. So depending on the day of the week and the time of day, there might be only one or two that can work for another hour, so any lame horse might put me out of luck... And then, there is an added complication: a couple of them are used by the showjumping and eventing teams, and might be having a rest day before a competition. As I improve, though, another three or four horses will become available for me.
And it's true for most businesses, that you don't want to work your plant and machinery or your staff at anything near to 100% of capacity for long periods of time.