You could argue that monopolies only increase their own profits in the short-term. Once a monopoly is established, it has no incentive to improve itself, either by increasing revenue or by decreasing expenses. Thus, it loses the opportunity cost/benefit of efficiency gains over time. It becomes a cash cow, but never really grows. The source of
real wealth in capitalism is all based around growth, and not a stagnant company that nets $x in profits year-over-year-over-year.
Plus, when a competitor does come out, monopolies tend to get squashed if they don't buy their way out of the problem (which further reduces profits...).
Saying they have no incentive to improve and then saying how that will cause them to fail is contradictory. If it will cause them to fail, then that's clearly incentive for continuous improvements so it continues growing and competitors simply can't compete.
World of Warcraft has done this in the MMORPG category. They came in, developed a huge player base, and basically got a monopoly on the genre. Yet, they constantly fix bugs, provide excellent customer service, and add new content. That way, when new competition comes out.. they simply can't compete; all the while WoW continues to grow as the game gets better and better to keep people sticking around and new people to join.
Also cash cows are good, I don't see where you're going with that. The idea behind a cash cow is that the growth rate has slowed so much that it can just be milked until people are done with it, generating high income with low expense. For example.. the iPod is a cash cow for Apple. They've milked it to fund other projects like the iPhone and iPad. Continuing to improve the iPod is pointless at this point as it's market is saturated and Apple can still branch out to acquire growth in other markets. If there is still growth to obtain, it stays a star and the company keeps pumping money into it.
Not saying the BCG Matrix is the best thing ever btw. But you brought it up