I noticed that you said Smartphones as a platform indicating that iOS is not the only player in town.Maybe you have answered your question yourself. This is not some niche market anymore, with games, camera apps dominating the download charts. Smartphones as a platform have become substantial infrastructure of large parts of the digital economy. It's only consequential, that at some point lawmaker decided, that this part of the economy needed some rules that govern the access to these platforms. This is what the Digital Markets Act is. Other countries will follow with similar laws.
indeed users of smartphones do have a choice
android with its open platform or iOS with its closed platform.
Apple only becomes a gatekeeper once a user has CHOSEN to purchase an iPhone as their smartphone.