You're thinking about this like it's just a single uniform endless pasture of gray-green moss. But you have to recognize all the moss is competing for space and resources.

So you've got 40M years of different kinds of mosses all developing novel evolutionary strategies as they try to one up one another. Just a rainforest of mosses, with an uncountable variation of shapes and colors and compositions.

Moss bushes. Moss trees. Hanging mosses. Floating mosses. Dense spongey moss. Brilliantly colored moss. Poisoned moss. Cannibal moss. Stinging moss. Velvety moss. Venus Fly Moss. Moss of a thousand different color variants.

And every few hundred years, you get a new moss meta strategy for being the best kind of moss that pushes all the other moss out. Played across 40M years, it's this big squirling fractual of warring moss tribes, until finally another organism figures out the optimal play on all moss and then it's over as fast as it started.