the biggest barrier to competition is not industry, but industry convincing government entities to erect barriers to competition such as zoning laws, environmental laws, and other laws that make starting a new company cost-prohibitive.
A large company cannot buy up a smaller company if the smaller company refuses to sell.
A large company cannot legally keep a smaller company from operating or existing. Only with the goverment can that larger company hinder the smaller one.
Example: I own every gas station in my town. An entrepreneur sees a niche for a new gas station. I can only stop him if I get the city council and mayor to pass laws making it very expensive to install new gas tanks or get business zoning for gas stations. Even if I cut my throat by lowering the price of gas to where I can't make a profit, he can still offer services, service, or other things that make his business attractive. That's how mom and pops survie against wal-mart -specializtion and customer service.
as far as pollution. If you are harmed by a company's pollution, you can take them to court, and a company does not have a vested interest in being sued all the time.