Ironically, the Apple App store was found to not be a monopoly. Or the playstation store, or xbox store for that matter.

In fact the legal system punishes you for having an open platform. The very reason the author can leave the play store is the same reason why the play store is a "monopoly". So sure, maybe the play store is a monopoly with some rigid definitions that lack nuance, but lets not throw away logic here.

If Apple/Google v Epic is what's on your mind, it was not having an open platform that resulted in the judgement. More what they did beyond just operating the platform. Being a monopoly in the US is legal, abusing market position less so. Maybe that lacks nuance and is a bit rigid, but it's the law in the States and what courts are bound by.

Something like the DMA and gatekeepers exist in part exactly because of the status quo and entities like Apple. I personally see the gatekeeper approach as a solid base, but know that it can be quite unpopular within the industry.