[gojomo]

The "or" in the question is a false dichotomy. Many historic examples of monopolistic abuse originated from market power that was initially acquired via excellence in innovation or execution.

Also many anti-competitive behaviors, like certain pricing/tying/bundling/cross-subsidization strategies, are perfectly legal for a small operator, but become illegal when a firm gains immense market power. And there is rarely a "bright line" for when a firm crosses that threshold, and the things it used to do become illegal. The firm pretty much has to wait for pushback via prosecution – actual or threatened – and of course will keep denying there's anything wrong with their habitual practices indefinitely, as a matter of corporate culture.

[narag]

Exactly. Also being a monopoly isn't illegal. That was talked about a lot at the time of Microsoft anti-trust case. What's illegal is using your monopoly position to get unfair advantage in other markets.