Ultimate control over devices you own should be a basic right. Apple's wanton abuse of users and developers via the control they have over their platform, and Google's nipping at their heels, should be evidence enough of that.
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.
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.