Yes, there is a co-evolution going on between plants and herbivores. Looked at the level of populations, rather than individuals, there is some give and take, but the relationship is largely a symbiotic one. Herbivores distribute seeds, either by their intestinal tract, or by their coats. The same goes for pollen. That act would be a good trade from the grass' perspective, but there's another important benefit that the herbivore brings: The creation and maintenance of habitat.

Perennial grasses (and clovers) that co-evolve with herbivores must be able to withstand being cropped every couple of years. Plants that can't deal with this cropping as well, can't compete in such an environment. This fact is why the plains were covered in perennial grasses (and clovers) but not trees. If it weren't for the herbivores, trees would have taken over much more of the plains. If the plants are winning, they are winning with the help of the herbivores.

But your last two paragraphs are just about bang on. I won't quibble the details because it would muddy an very insightful point.