The more stable/secure a monopoly is in its position the less incentive it has to deliver high quality services.

If a company can build a monopoly (or oligopoly) in multiple markets, it can then use these monopolies to build stability for them all. For example, Google uses ads on the Google Search homepage to build a browser near-monopoly and uses Chrome to push people to use Google Search homepage. Both markets have to be attacked simultaneously by competitors to have a fighting chance.