I'm not sure how we could enforce that, but maybe the law could stipulate that a certain minimum percentage of users must use 3rd party app stores, or use web apps. They should pay a fine if less than say 5% of apps are distributed outside the app store, or if less than 5% of people use a 3rd party browser engine.
If a competitor wants market share they have to build a better service. Forcing users to go with a bad deal gets the incentives all wrong and is actually bad for consumer choice.
However, instead of defining the market rules, the process has been more about competitors and companies (who’re not happy with Apple’s success) trying to take a chunk of their business.
An iPhone is not a general computation device, it’s not an open ecosystem. Neither PlayStation, but there’s enough competition in the gaming console sector so nobody comes up with complains about not being able to install any app they want.
Edit: spelling and clarity.
And yet it's many people's primary computing device. That's exactly the problem.
As a historical example, consider telecommunications. Phone networks were "natural monopolies" for many decades, and people must have found it hard to imagine any other way back then. Without regulatory intervention enforcing competition, we'd probably still be paying double-digit cent amounts for long-distance calls.
Apple entered the smartphone market as an almost bankrupt company and replaced the giants of its day like Nokia and Blackberry. And some other company will replace them someday. The DMA discussions started really good, I had really high hopes of it, but it got almost hijacked with various companies to publicly negotiate contracts with Apple and not for the public good as a market regulation.