The EU seems keen to enforce this. The DMA set to come in will force Apple to allow third party app stores and side loaded apps to iOS, where it's not currently done
I think regular anti-trust laws would be the way this would be fought on the desktop.
macos as a whole doesn't have enough users in the EEA to fall under the purview of the gatekeeper section of the DMA, unless the "app store" counts as a single thing. Which it kind of does now that apple has unified it between iOS, iPad OS, and macOS.
Windows S versions that only allow software from the microsoft store haven't drawn any ire from EU countries, AFAIK.
macos as a whole doesn't have enough users in the EEA to fall under the purview of the gatekeeper section of the DMA, unless the "app store" counts as a single thing. Which it kind of does now that apple has unified it between iOS, iPad OS, and macOS.
Windows S versions that only allow software from the microsoft store haven't drawn any ire from EU countries, AFAIK.