Yea, I agree.
“Progressivism” is a sport for the young. In the 70’s, it was the early Boomers (‘46–54 birth dates, spent their 20’s and early 30’s in the 70’s) that drove that “demand”, and as you point out, skewed party politics to the disaster of 1972.
Now, what are we seeing? The children of the boomers (so logically the second largest generation in our history) millennial (82–96) are now in their 20’s and early 30’s. They are forming households late because of the events of 2008, so they are just starting (in the last two-four years) their “mugged by reality” shift which turns progressives into liberals, liberals into centrists, and centrists into conservatives.
I expect we are watching history repeat itself. Whether or not the electoral mugging of McGovern will be repeated is anyone’s guess, but I completely agree with your assessment that if the face of the Dems becomes “progressive”, an uncomfortable reckoning for liberal and centrist Dems is at hand.