Although ever since horses were first domesticated thousands of years ago, horsemen realised the importance of protecting their animals feet. On hard or rocky terrain, shoes protected a horses hooves from cracking or wearing down faster then they could grow. In soft terrain like the farms of Ireland – shoes help the horse gain a good footing as well as helping prevent disease and degradation associated with damp feet.
So, say you don’t make great demands on your horse, you are a stroller or occasional competitor. You notice that your horses hooves are regularly long when the farrier arrives, necessitating a trim before re-shoeing, then perhaps you can do without shoes. However, you must attend to hoof hygiene to prevent lovely diseases like thrush. Oiling the hoof fits in here nicely. The horse may still need trimming occasionally but in a slightly different way, not to take a shoe, but to create the natural wild shape.
The nearest thing we have to hard or rocky terrain would be the roads and you can crack and wear down hooves excessively there. I would contend that feet could be acclimatised to road work if given a chance, gradually introducing them to it. Think of the first time you pull your own shoes off to walk on beach pebbles. Very prickly, but after a day or so you get to be running over them without fear!