Actually, they are not real monopolies, but rather collectively they make up an oligopoly where the companies compete at the margins, mainly in the form of product differentiation, They are successful as long as they innovate. Why would they not continue to do so? On first thought, because they run out of innovations. But is that likely?
I don't think so. The problem here is that further innovation gets blocked by patents and copyrights so that big companies like Apple and Microsoft use their intellectual property to ward off new competitors by building an IP fence. There is competition now and it will continue so long as the industry can continue to innovate. When innovation stops, the oligopolists will be in full control and the need to do something about the industry's IP will become pressing.
Politically, it seems hard to do anything about this IP so long as innovation continues and the industry can argue it is constantly in flux. So far, the big players have been right.