Apple decided where the boundary of competition lies by deciding to bundle their phone, their operating system, their app store, and their code signing policies into a package deal. A thicket of copyright laws forbid you from unbundling any combination of these. For example, I can't legally run iOS on my Pixel 8 Pro. It is legal to run Android on an iPhone, except the hardware boot ROM is designed to reject anything not signed by Apple, and Apple won't sign Android. You can circumvent that by taking advantage of security vulnerabilities in the boot ROM[0], and that's legal, too, but you can't legally tell anyone about those vulnerabilities because telling people how to break copy protection is a felony.
This same logic applies to application software. It's legal to jailbreak[1] your phone to install non-App-Store apps, but illegal to provide the jailbreak tool necessary to install those apps. If Epic had decided to release a jailbreak to install the Epic Games Store, instead of releasing a hotfix for third-party in-app billing and suing for antitrust, Tim Sweeney would have been thrown in jail for violating 17 USC 1201.
In copyright law, there's a concept of copyright misuse, which is what the courts call using copyright to break antitrust law. Copyright allows you monopoly control over your work, but you are not allowed to use that control to maintain an illegal monopoly that's outside of the copyright bargain. In other words, you don't get to opt-out of forms of competition you don't like. Congress decides those.
So no, "build your own phone and app store" is not enough. And letting Apple dictate the conduct of their competitors is effectively giving them antitrust immunity.
[0] Which are, BTW, once-in-a-decade. We've had a grand total of two bugs that can be used to install alternative operating systems on an iPhone.
[1] Jailbreak bugs aren't as scarce as boot ROM bugs, but they've been getting scarcer over time. This is not because Apple is doing a good security job, but because the NSA is hoarding all the good bugs for themselves.
Apple decided where the boundary of competition lies by deciding to bundle their phone, their operating system, their app store, and their code signing policies into a package deal. A thicket of copyright laws forbid you from unbundling any combination of these. For example, I can't legally run iOS on my Pixel 8 Pro. It is legal to run Android on an iPhone, except the hardware boot ROM is designed to reject anything not signed by Apple, and Apple won't sign Android. You can circumvent that by taking advantage of security vulnerabilities in the boot ROM[0], and that's legal, too, but you can't legally tell anyone about those vulnerabilities because telling people how to break copy protection is a felony.
This same logic applies to application software. It's legal to jailbreak[1] your phone to install non-App-Store apps, but illegal to provide the jailbreak tool necessary to install those apps. If Epic had decided to release a jailbreak to install the Epic Games Store, instead of releasing a hotfix for third-party in-app billing and suing for antitrust, Tim Sweeney would have been thrown in jail for violating 17 USC 1201.
In copyright law, there's a concept of copyright misuse, which is what the courts call using copyright to break antitrust law. Copyright allows you monopoly control over your work, but you are not allowed to use that control to maintain an illegal monopoly that's outside of the copyright bargain. In other words, you don't get to opt-out of forms of competition you don't like. Congress decides those.
So no, "build your own phone and app store" is not enough. And letting Apple dictate the conduct of their competitors is effectively giving them antitrust immunity.
[0] Which are, BTW, once-in-a-decade. We've had a grand total of two bugs that can be used to install alternative operating systems on an iPhone.
[1] Jailbreak bugs aren't as scarce as boot ROM bugs, but they've been getting scarcer over time. This is not because Apple is doing a good security job, but because the NSA is hoarding all the good bugs for themselves.