Most likely, it would look like Asahi Linux, which has managed to reverse engineer and re-implement many parts of the Mac environment relatively quickly in Linux. If it works like the Mac does, we may see a project to make a custom ROM for iPhone (probably a fork of either a Linux phone project or of AOSP) soon after the responsive update, and within about a year of that, we might see it be fairly usable.
It is apparent however that the platform security model was engineered to allow third party operating systems to coexist with macOS in a way that does not compromise any of Apple’s security guarantees for macOS itself. Rumours circulating that Apple are actively hostile towards efforts such as Asahi, or that their security must be bypassed or jailbroken to run untrusted code are unfounded and false. In fact, Apple have expended effort and time on improving their security tooling in ways that only improve the execution of non-macOS binaries.
So if Apple complies with this new ruling in a similar way, we could expect to (eventually) have the same level of security on a third-party OS!
Most likely, it would look like Asahi Linux, which has managed to reverse engineer and re-implement many parts of the Mac environment relatively quickly in Linux. If it works like the Mac does, we may see a project to make a custom ROM for iPhone (probably a fork of either a Linux phone project or of AOSP) soon after the responsive update, and within about a year of that, we might see it be fairly usable.
Oh neat: https://github.com/AsahiLinux/docs/wiki/Apple-Platform-Security-Crash-Course#apples-unspoken-agreement
So if Apple complies with this new ruling in a similar way, we could expect to (eventually) have the same level of security on a third-party OS!