> The problem with that approach is $popular_social_media app comes along and coaxes users to relax said privileges "because reasons" and before long there's a signigficant proportion of users who altered the security model of their device without understanding what is going on.

And the problem with a single signature authority, as we're seeing in China, is being murdered by an illicit state, or undergoing active discrimination in many other ways even in most western countries.