alt Hacker Newswhat you are describing sounds like an oligopoly not a monopoly. A company cannot realize monopolistic benefits without > 50% control of a market. That's not to say there aren't benefits to a large market share, but they are different and we have different terms for them.
US law sets the breakpoint for when a company is considered a monopoly at 30% of a market. It's never actually enforced, of course, but may be worth noting.