Also it's not that system was throwing UAC prompts on users, applications did. Ever since XP there were rules around what files go where, how installers should

Also it's not that system was throwing UAC prompts on users, applications did. Ever since XP there were rules around what files go where, how installers should behave, etc. but people chose to ignore them (because they could). When Vista started enforcing certain things, elevation dialogs happened. What made MS in the days past (app ecosystem) broke it when quality started to matter.