Stop parroting the corporate propaganda that put us into this stupid situation in the first place. Having root access on devices you own should be a fundamental right, as otherwise it's not ownership.
These restrictions extend outside the particular device. It must also be illegal as a commercial entity to enforce security schemes which involve remote attestation of the software stack on the client device such that service providers can refuse to service clients based on failing attestation. Service providers have other means of protecting themselves, taking away users control of their own devices is a heavy handed and unnecessarily draconian approach which ultimately only benefits the ad company that happens to make the software stack since they also benefit from restricting what software users can run. Hypothetically, they might be interested in making it impossible to modify video players to skip ads.
1. Devices should be allowed to display a different logo at boot time depending on whether the software is manufacturer-approved or not. That way, if somebody sells you an used device with a flashed firmware that steals all your financial data, you have a way to know.
2. Going from approved to unapproved firmware should result in a full device wipe, Chromebook style. Possibly with a three-day cooldown. Those aren't too much of an obstacle for a true tinkerer who knows what they're doing, but they make it harder to social engineer people into installing a firmware of the attackers' choosing.
3. Users should have the ability to opt themselves into cryptographic protection, either on the original or modified firmware, for anti-theft reasons. Otherwise, devices become extremely attractive to steal.
I'm alright with limiting liability for an unlocked/customized phone (for things that happen from that phone) - but that's a legal/contractual thing. For that to work, it's enough for a judge to understand that the phone was customized at that time - it doesn't require the app to know.