I think after the 1970's parents & grandparents no longer cared. Me and wife were discussing the time recently. We figured it was a combination of three to six different decades rolled into one.
You got stuff coming to the 70's from: 1920-1930's, 1950-1980's. The seventies really didn't have its own identity aside from disco. Everyone is still trying to understand disco, or trying to forget it if they can.
You had fifty years, possibly more wound up tight into one decade. The seventies brought aboard equality to a great level. It didn't matter if you liked Blue Moon or Highway to Hell, everyone was someone and all the someone's were no-ones.
All races and creeds seemed to get on then too, there was no discussion of religion. That was up to each person and nobody cared like no one cared if you slept half dozen to a bed. The seventies simply upended and destroyed convention.
Hemingway wrote about a lost generation in the thirties. Well it got found and lost again in the seventies. Parents and grandparents stopped caring because the children ran off following the Piper's tune.
I think that's why we get no more manners, thank you.