It's a question of whether you want regulatory intervention preventing the possibility of there being "closed" systems in addition to there being "open" systems

It's a question of whether you want regulatory intervention preventing the possibility of there being "closed" systems in addition to there being "open" systems. If you prevent Apple from going their "closed" route, you lose the possibility of any benefits that might come from such a different model. When people say "let the market sort it out", they mean that if Apple's model puts out a significantly worse product than the more open models, they'll be forced to open up or die. So it's a question of whether all platforms have to be open: this would arguably lead to increased competition within each platform, but with less distinction between platforms and would prevent the possibility of exploring different models for whole platforms (and letting those models compete).