Evolution

improve to adequacy. Once a trait is developed enough it will level off. For example let us use the idea of elephant tusks. Longer tusks mean it's easy to dig, meaning it's easier to find food during the dry months, meaning better chance to breed. Once all the elephants have tusks long enough it's no long a selector and has no bearing on ability to breed unless the females actively look for longer tusks. At this point, so long as the females don't become selective about it the trait will just level off and all the elephants will have adequate tusk length with little variation. So long as the environment doesn't shift, the tusks shouldn't either.
I used tusks because you can see a lot of examples where the tusks got too specialized and screwed them over.

Would the tusks becoming stronger or denser really be useful? It's extra weight in an already horribly unbalanced position. Is it worth the extra muscle mass it would require? Most likely not.