they can't do anything they want because they still face the risk of replacement. Monopolistic competition is the dominant form of activity in innovative economies, temporarily outsized profits scale enable innovation and long term planning. (which benefits consumers)
MySpace in 2007 had 80% of social network traffic, turns out it could not do what it wanted. Advanced tech economies generally consist of monopolists replacing each other, not bazaar economies.
MySpace isn't a good example because it was early in a growing and changing space. Better example are Windows, duopoly of iOS and Android.
It may still be possible to beat a monopoly by being extremely good and aggressive right on time when your competitor is napping. e.g. Firefox and Chrome vs IE. If IE team hadn't been dismantled and added tabs and other UI features, worked on performance and security improvement, we may have been using IE 25.
> they can't do anything they want because they still face the risk of replacement
Not necessarily, especially if the switching cost is too high. For example, if Microsoft does something monopolistic, which they had done, people aren't going to switch to Linux. Therefore the government had to step in.
MySpace in 2007 had 80% of social network traffic, turns out it could not do what it wanted. Advanced tech economies generally consist of monopolists replacing each other, not bazaar economies.