The market pressures faced by firms (which, I'm assuming, are responsible for anything you might view as "firm efficiency") are similar to evolutionary pressure

The market pressures faced by firms (which, I'm assuming, are responsible for anything you might view as "firm efficiency") are similar to evolutionary pressures faced by any living creature. Evolution doesn't create "perfect" or "efficient" creatures, but predominantly creatures that are just "good enough." If you can live long enough to reproduce and pass on your genes, you're in the club. An animal with no natural predators, for instance, may get bigger and less adept at fighting off potential threats (after some generations,) which is kind of similar to the stagnation of a monopoly.