Still, the problem is that if you go this way, you'd have to put almost all useful functionality of a modern phone on a smartwatch, at which point you could just ditch the phone.
It's not just one tiny use case that's pushing us down the road of increasingly locked down devices. It's most use cases - because no matter the service, it's more profitable for the provider to control what you can and cannot do.
I don't think that's actually true? That's like insisting all useful functionality would have to be moved to a smartcard/yubikey/bitcoin hardware wallet/TPM etc. The main reason this is an issue is to prevent emulated hardware tokens. If you can disable secure boot, you can emulate secure elements and then things that others (i.e. your bank, government, etc) believe are carefully controlled secrets are not.