What can be done is prevent them from becoming near-monopolies with a captured audience.
Cory Doctorow calls this "enshittification" - his term.
The basic idea is that in the beginning, new companies are all about their users, luring them away from competitors, providing the best experience for cheap, with great service. That captures them. Think amazon, underpricing everyone, shipping for free, taking all returns, making a loss for years until everyone just uses amazon to search for products.
Once you have their market, you shift gears, from serving users to serving your financiers, selling the user data/user attention/user experience to them: Amazon selling ranked listings in the search that all the captured customers now use as a default search engine. This makes every seller use amazon to sell. You have to, to reach the captured audience. And you have to pay amazon to be seen.
And finally, using this machine you built, with captured customers and captured sellers to squeeze the most of it. Amazon copying the best-selling items and top-ranking their own products, without of course charging itself fees for either the listing or a cut of the sale, as the do with third parties.