How would that help anyone? The unlocked bootloader on the Apple Silicon Macbooks still hasn’t resulted in a truly viable alternative to macOS. The iPhone is much, much harder. And why would carriers trust the result?

You’re missing the point. Hackers (in the tinkerer sense) just want to program their own devices, with full access to the capabilities of the underlying hardware. Apple could obviously allow that if they wanted to, in a convenient way that doesn’t increase the attack surface for malware.