Monopolies rise up in the scenario you describe only in a static marketplace. Very few, if any, markets are static. There is constant change. Well healed competitors from other markets will enter a market where they see monopoly pricing power and begin to compete to grab a piece of a high margin business. Disruptive technologies or methods are also in play. In order for a company to successfully buy it's way into a monopoly, it requires that all other competitors (and potential competitors) either acquiesce to being bought or not enter the market after the monopoly has been created.
Certainly there can be barriers to entry, but those barriers will not be permanent.
A pleasant fiction, but a well-operated monopoly will move to crush nascent competition by undercutting them in price or securing their supplies—and thus their ability to produce at all—at prices that cannot be competed with by a smaller entity.
This isn't to say that it wouldn't be possible to defeat a monopoly eventually, just that it's not as straightforward as you make it sound. Indeed, historically, monopolies seem to have required government intervention to be dismantled (Standard Oil, AT&T, etc), though perhaps that's simply defining monopolies as those entities that the government dismantled as such.
Regardless, I think this is one of the two “silver bullet”-style myths that likes to float around the area of government regulation: one, that less regulation is always better, the other, that more regulation is always better. In essence, a refusal to acknowledge that regulation is and should be in a fluid state, adapting to what the market is doing at any given moment—and ideally ahead of the curve, rather than reacting after financial or other disaster has already struck.
You are assuming that competition will only come from smaller entities. My comment was simplistic because each scenario is different and I was speaking in the general case. Standard Oil had lost a lot of market share by the time it was broken up. AT&T existed as monopoly because government had heavily regulated the telecommunications industry.
Certainly there can be barriers to entry, but those barriers will not be permanent.