And even that blocking doesn't matter much anymore, because the owners of all those 3-5 companies are starting to be the same entities. Because the same people/organizations own all of the companies, they no longer care which one is currently winning and so won't push them to compete against each other as much as they would if the ownership didn't have any overlap. They'll push for policies that allow those players to exist together without stepping on each others toes.

Not at all, you see this pattern in markets that regulators ignore too. It's a human limit. You end up with 3-5 competitors because the incremental value of yet another competitor becomes nearly zero very fast. People have very strict limits on how much information about a specific market they care to remember. To even have 2-3 competitors they have to put a ton of work into differentiation and marketing, otherwise people forget they exist.

Sometimes you do see more, and it's indeed because regulators are stopping mergers. The airline market is like that. But it's a mistake and the regulators should back off. It's not helping people to have a gazillion undifferentiated brands, at some point it's just wasteful duplication.

The idea that every market would contract to a single player if not for the benign wisdom of the regulators is a trope. Regulators are tiny compared to the number of markets that exist, but competitive markets aren't the exception, they're the norm. You see it even in very sticky locked-in markets where regulators deliberately decided not to intervene, like operating systems and browsers (Justice Dept effort to break up MS was cancelled).