You must be thinking this is all wrong. I completely understand and agree with your sentiment, but we're talking about what the contract says.
I'm not allowed to install any software I want on my car's computer, the platform belongs to them. They don't provide the tools, libraries, the know-how, or even sue the people who share it online. And similarly, according to Apple's EULA the devices cannot run any app that is not approved by Apple and they can even revoke their approval or even disable the phone.
Those were the license conditions the hardware sold under, which sounds very user hostile. Regardless, nobody has to buy their products, they chose to buy it because the benefits it provided surpassed the limitations. When Spotify created their developer account they knew what the limitations were as well. This isn't an open platform. One can sue Toyota to get access to install Spotify to Corollas and get another 500 million customers, but that also wouldn't work either.
The only thing that can stop Apple is people not buying their products and developers not making apps therefore reducing the value of their ecosystem. Only then they will by themselves would open the ecosystem, which they should've done 5 years ago.
Regarding the EU forcibly taking stuff over. Well, if push comes to shove, do you think the US would allow a 3 trillion dollar American company to be bullied, go after European companies or would they react in a really unpredictable way?
Apple devices are successful because they provide a great value. They didn't just sell the hardware like Nokia did, they kept delivering software updates and spend billions of dollars sustaining the ecosystem. The limitations were put to improve user experience, for example they didn't allow apps to run continuously in the background so that users can have all day battery life. The high level of control they have allowed them to provide greater value than other ecosystems which brought more users and so on. This requires continuous work to keep it running and they're entitled to be paid for their work.
And again, nobody has to buy their products, you can buy other products and install whatever software you want on those, and do whatever you want there. Android has a bigger marketshare and some people still use Nokia or Blackberry.
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A digital marketplace consists of everyone that participates in the digital economy not just Apple. All the websites, service providers, apps, hardware manufacturers, users, companies, and their interactions.