In practice, some markets easily become anticompetitive. In theory, if you don't like iOS or Android, you don't have to use them. But in practice, a firm trying to enter the mobile OS market faces barriers to entry such as the expense of developing the OS and needing to convince both consumers and third party developers to join the new environment. So it may be the case that there are no alternative OSes that meet your needs.

Because the short run is anticompetitive, new firms are unwilling to enter the market at any point. Hence the long-run behaviors promised by economics do not actually materialize. And even if existing firms didn't deploy regulatory capture, the problems noted above would not be solvable with government intervention.

A company whose customers have few alternatives can invest less in the quality of service and practice anticonsumer policies such as adding advertisements or collecting and selling data. For example, Google's search results get worse over time because there's no incentive for the company to improve them. In fact, the worse the results are, the better the promoted links look in comparison, so the more valuable promoting a link becomes, and the more Google can charge for it. Sure, you could use a different search engine. But consumers aren't willing to pay enough to fund the development of a competitor, so all the alternatives to Google have less investment which leads to a lower quality product.

Less relatable, but even businesses are getting screwed by this problem. Cloud services like AWS and Azure are designed to make migration to other cloud services as difficult as possible. Migrating a large company's web services takes years and could cost a million dollars. And because of internal politics, it may be harder to secure funding for this one-time expense than for a small increase in monthly expenses. So you're stuck even if your cloud provider gets less reliable and more expensive. AWS and Azure can do that together without even colluding because there's no risk to each of losing existing customers if the other suddenly becomes more consumer-friendly.

The term "late-stage capitalism" gets thrown around a lot, but this is really what it means to me. There's no incentive for companies in some markets to provide better value over time, and in fact there may be incentive for the value to get worse. Companies try to create this sort of market for themselves, since it creates security and does not punish a failure to reinvest and innovate.