State, for example, doesn't leak. You never worry about the fact that it's a function `s -> (s, a)` and it gets optimized away like nuts.

In the unlikely event that there was a change to the implementation of the State monad, or to one's compiler, such that the State monad was not optimized away, it would be disruptive to users. It would probably be treated as a bug, even if the only change in behavior was in additional CPU and memory usage.

This is a corollary of Hyrum's Law.