A company that sustains excessive profit margins (over 20%) over more than 3 years is often a bad sign however, because it indicates an uncompetitive market. In a perfect market profit margins always tend to zero.
I agree but not entirely. By the way, where do you pop such numbers from?
It is true that if a certain market remains very lucrative for a long time, it is quite suspicious. One may say that there are artificial (coercive) barriers to entry in such market, protecting those who are already in. Banks are a good example. But that's not always the case... there are historical cases of companies capable of maintaining competitive advantages over everyone else for years.
Saying that in a "perfect market" profit margins tend to zero is too strong, and not true... if a particular market start to have lower profit margins than others, providers will migrate out of this market to those more profitable, what can stop the decline of the profit margins of this market. The "total average" must be positive as well, it's not a zero-sum game.... profit = wealth creation, and people are always looking for ways of creating more wealth. Free markets are good to everyone, providers and customers...
Instead of saying they "tend to zero", you could say they "tend to an equilibrium", which, of course, will never be reached, since society is always changing.