At two ends of the spectrum, there are monopolies and then there is fragmentation. It feels like, hypothetically, massive consolidations of power/monopolies can be broken up with regulation, and that would be better for competition. But if in practice consumers only ever go back to the storefront/service that is functionally superior and that most people use, would it make enough of a difference in the long run? If there was finally enough regulation to allow alternative app stores on iOS, what exactly would motivate enough consumers to care about the new alternatives? (Knowing that timed exclusives on PC tend to generate considerable pushback.)
There is no option to force consumers to use the other products with regulation. They will only be driven by the competence/exclusives/network effects of other platforms. Maybe the end outcomes of having multiple options will simply be a partial result of how competent the other development teams are in comparison to the larger and objectively better platforms, in spite of the fact that consolidation is considered bad in principle. It seems to be getting harder and harder for even large entities like Bethesda to create successful alternatives in spite of all the resources they have.
The blowback that multiple companies have gotten for choosing to release games as Epic Game Store exclusives and Discord becoming gradually ubiquitous are what I would consider examples of this phenomenon. Average people simply want to use the larger, better platforms, and they aren't convinced to move when alternatives exist.
Because of the sheer size of platforms like these, success might just be defined in terms of momentum and ubiquity by now. You would be up against convincing a subset of millions and millions of people that you can deliver a functionally superior platform, and if their friends decide not to move, then they would have good reason not to move also. Developer cuts tend to be criticized, but I would think that most users of Steam wouldn't even be aware of such issues since they stay local to the development side and don't impact their experience, and so they keep using Steam anyway.