If we remove all the ferns and horsetails from the ecological equation would grass be the only candidate to fill the void? If not, then what else?
If you find yourself in an environment where there are a lot of ferns and horsetails, there is likely to be a lot of moss around too. Now mosses don't grow particularly tall ( though there are some larger ones ) but they can grow on many different surfaces without needing much soil and survive well in very boggy ground.
In an environment without ferns and horsetails it seems quite plausible that you might see a divergence of mosses filling those ecological niches.
In the short term plants that are more similar to grass are likely to move in to fill the void initially, but even if that were not the case eventually some plant would move to fill the niche. There are a wide range of plants including gorse, heathers, etc.
For example take a look at this picture:
There isn't much grass, fern, or horsetail in this picture but there is extensive low level plant cover filling a similar role.