People that think this is unacceptable are not remotely average users. Average users benefit greatly from their pocket appliance not being a full fledged computer.
Fundamentally, it is a trust issue. Why should I be forced to trust Google or Apple has my best interests in mind (they don't)? That is not ensuring 'device integrity', it's ensuring that I am at the whims of a corporation which doesn't care about me and will leverage what it can to extract as much blood as it can from me. You can ensure 'device integrity' without putting any permanent trust in Google or Apple.
You are not.
It's certainly convenient in this modern world to pay for and use one of their devices though.
Considering market forces are against it, I believe the only practical way to accomplish this in the long term is for this to be a right that is enforced by legislation. I don't think it is even far from precedent surrounding first sale doctrine and things like Magnuson-Moss, that the user should be the ultimate one in control post-purchase, it just takes a different shape when we're talking about computing technology.
No one is forcing you to buy a particular device.
True. But society in practice requires a smartphone with one of two operating systems to live a normal life without significant efficiency losses in your day. Now all phones with both of those will be completely walled off. You'll be forced to participate or make your life a lot less convenient.
Surely you wouldn't defend absolutely anything happening to say roads just because you're not forced to drive, technically speaking?