alt Hacker NewsIt's totally unfeasable for those using stock deviced. Refusing to upgrade takes lots of attention even from experienced users like developers. Regular user just doesn't have any chance to avoid accidentally clicking or intentionally accepting the annoying permanent notification to upgrade OS.
It's the norm for the huge number of users with devices where there is no newer upgrade available from the original manufacturer. Back when Android was great(tm) there were far more of those than today.