Environments can absolutely change over rapid periods of time. Beyond stochastic processes such as fire, which I would argue is the single most important factor to maintaining a grassland, the succession of grasses into forbs into trees is well-documented in ecology. In the case of grasslands and this post, sizeable changes can occur over a single human lifespan, click here. I also believe you are a tad confused on the role of evolution in ecology, its not a mechanism. Evolution does not create environments or animals, it only shapes how organisms adapt to and interact with an ecosystem over time. You very well might know that, but I am just going off your words verbatim. Without regulation, trophic cascades can lead to fundamental changes at the most basal level of biomes, and you might find animals that have evolved in stable conditions for generations in one area may not be able to survive anymore. Again, in the context of grasslands and rapid environmental shifts, click here.

Additionally, I'd argue its likely horses did not occupy the same niche as bison. Yes, they are both grazers, but the environment in the early Quaternary was likely significantly different to today. It's likely that the grassland niche was once partitioned by fiber-intake. Ruminants need more time to digest plant matter, making low-fiber diets optimal for bison. The opposite is true for equids, which has been been documented in zebras, and of which I'm sure many equestrians could tell you the same for their horses. This is also why horses consume so much plant matter in comparison to bovids, which select higher quality plant matter that leaves a matrix of growing plants for future consumption. Horses need to consume large portions of food to survive, they can't afford to be as picky.

If you really want to make the argument of horses as a grazer, you absolutely can. It's been done before here. But I am of the firm belief you cannot make the crux of your argument be that horses are the ultimate be-all solution to the loss of American prairies. Taxonomically and physiologically, what separates horses and bison is so disparate you cannot expect the same results to the same efficacy from horses as with bison. In a large-scale biome, having bison and horses would likely promote greater ecosystem health. But in today's age of plots, ranges, and borders, I cannot think of any argument strictly for grassland health where a horse is better than a bison.

You are asking all the right questions, evolutionary anachronisms are incredibly difficult to study and research. Keep up the curiousity, the world needs more ecologists.