The only scenario that demonstrates the existent of such a monopoly is farcical to the extreme. It would require a company with a never ending supply of revenue allowing it to purchase every competitor at the price their owners value it. As the company spent more and more on buying out competitors, the costs of those purchases will have to be paid through a rise in cost. As the costs rises, in comes room in the market for competition. Remember, monopolies in a free market can exist only if the monopoly can sell a good at a cheaper price than any competitor.
Just google Government-generated monopolies. It's a term so common it has a wikipedia page. Then consider this. You agree that there are inherit costs in a business for agreeing to regulation, correct? That includes all sorts of regulation - including acts like the Americans with Disabilities Act. In order to abiding by that regulation, you must take certain precautions that you may not otherwise make in a skeleton budget. Now the costs can be high or low, but money is spent on certain objects that would otherwise have not been bought. That's extra costs of entry. That inherently decreases competition.