Uhm, what evidence do you have of this? I mean yes, I understand the theory, and have heard it about a billion times, but what *evidence* is there.
The person you quoted gave a couple of good examples of non-governmentally enforced monopolies. Standard Oil. US Steel. Neither of these were monopolies enforced by governmental action. Both caused terrible drains on everyone, especially the workers (I doubt the people lining the gates begging for a days wages would have said that everyone was happy) who were essentially forced into servitude by the situation. But this is getting offtopic, I'm still looking for evidence.
Barrier of entry to market is not as trivial as you make it sound. Yes it may sound "fair" that it's difficult to enter a profitable market for late comers, but it doesn't help your point that monopolies are balanced by potential rivals if the barrier to entry is so high that competition is almost impossible.
Look, if I have a business harvesting moon rocks and you complain that I'm charging too high of a price or that it creates a terrible drain on the workers then start your own company. If you can't find investors willing to risk their money then what right do you have to say that the prices are too high? All prices are too high since consumers would like to get everything for free but when dealing with economics we have to evaluate actions, not words.
Let's take oil for example. It's rare and hard to acquire. Therefore, the prices they charge reflect this. If the price of oil is too high then people will switch to other sources of energy. If they are still buying it, it doesn't matter if they are grumbling about the price, clearly the price isn't too high, otherwise, it wouldn't be selling. The same applies to any other natural monopoly. Free market monopolies aren't "bad" as long as you get rid of the idea that you should be able to get everything for free. The prices will always approach what it's actually worth.
As a potential counter example, look at microsoft. An inferior product at hugely inflated prices from a terribly inefficient company that is essentially a monopoly due to legacy support requirements and an almost impossibly high barrier to entry at market.
That's just opinion, nothing more. Perhaps the software itself is inferior but you can't consider it in a vacuum. You need to consider human capital, how many people understand it, use it, can program with it, what kind of support there is for it, etc. If Linux was so wonderful, it would have taken over by now. Counterexample, Mozilla Firefox. Internet Explorer had a monopoly but now it's losing market share daily. Firefox is king these days.