I see nothing wrong with a monopoly which is gained purely by the free market, because that's more efficient in serving the consumer.
If Hank Aaron is the best baseball home run hitter, we shouldn't break him up and call him a monopolist and tie a weight on his arm or something.
Say, you're evil because you're better than the other home run hitters.
On the other hand, if he gets his home run a-hitting advantage by breaking the arms of his teammates or the other team, then something should be done about it.
and usually the antitrust division is the on-banking category instead of the other way around.