Easier with a duopoly for sure (if there's something restricting new entrants to the market, that's a big problem in its own right), but even so, it's a prisone

Easier with a duopoly for sure (if there's something restricting new entrants to the market, that's a big problem in its own right), but even so, it's a prisoners dilemma. They are better off cooperating, but if one defects first they can screw the other one, so it relies on trust.