the moment any of the major miners merge, they can conveniently control the direction of price and nothing anyone would be able to do about it because there wont be any regulator to forestall such move just like the case it would have been in the business world where merging of two firms is likely to cause any form of monopoly of the market they operate.
If only two merged and wanted to create a monopoly, others would also merge and create an oligopoly, in which we would most likely see Bertrand competition. Those two players would undercut eachother resulting in the exact opposite of what you described. The price would drop until one player left the competition resulting in a monopoly. Then the remaining firm would raise the price until the profit was high enough for another firm to enter this market. Because those firms most likely don't trust eachother, they won't merge together.