Who is saying that enforcing companies to open their systems to competition is making them mediocre? Maybe if that's the end result, they should put more time into designing systems that wouldn't become mediocre just by allowing third parties to do things with those said systems? We need to stop defending corporates for abusing their monopolies.
Interoperability is not free. One of the trades it brings is a notably lowered ceiling in terms of tightness and capabilities, and this persists no matter how many man-hours are poured into engineering the systems that enable it.
The Linux desktop is a great example of this at play. While it's technically worked for decades at this point, it's been a constant struggle to make it a high quality, thoroughly polished experience end to end and that's partly thanks to the unavoidable friction and gaps between layers that comes with interoperability and tens of involved parties.