The EU had a study that indicated that you need at least four competitors before price competition appears. Two or three act like a monopoly, almost automatically. The free market thing requires multiple players to work. So mergers that bring the number of competitors below four should be prohibited.

The US is down to three big banks, three mobile phone companies, and three drugstore chains, and they don't compete on price much. If an industry gets below four, maybe the companies should be given the choice of breaking up or converting to a common carrier or regulated public utility.