Imagine it's the 1800s and you wanted to develop technology to let anyone listen to music anytime. You'd have to invent the computer, an operating system, and the internet (some means of distribution), then invent Spotify. (Or maybe not all those things in their entirety, but just enough to support the Spotify's functionality.)
Since it's 2019, there are platforms that provide the operating system and means of distribution, and Spotify merely has to deliver an app that floats in high-level-land. They then complain that that platform, who did all the work Spotify didn't have to, isn't being "fair." What does that mean? What are they rightfully owed?
Why doesn't Spotify create their own OS and App Store, and develop/distribute Spotify there? Because it's inconceivably hard? Surely that's why the people who did it get to set the rules.
This platform ownership piffle Apple has sold for eons is pure horse shit, and the EU regulators will rule as such. Simply put, a fridge manufacturer cannot control what items get stored and cooled in the fridge, after the customer has paid in full for both the fridge and the item. And the fridge manufacturer certainly cannot restrict items only sold though it's own stores