Who says American voter turnout declined starting only in the 1960s? It was lower in the 1950s, and started its major decline in around 1900, remaining in the 50-60% turnout range for presidential elections for most of the last century (the midterm election turnouts were lower). So I'm afraid that can't be blamed on the Hippies. See the links below for the numbers.

Progressives might be as conspiratorial as Trumpians in some respects, but progressives don't run the DNC whereas Trumpians do run the GOP. That's the difference. Leading Democrats are mostly technocratic centrists.

Rig the system so that no Republican can ever hold office again? What are you talking about? The system has been rigged to give Republicans more of a voice than they deserve, to give undue weight to the opinions of smaller, more rural states, which have more senators and Electoral College votes per capita than the larger ones.

In any case, why should a party that's become a suicidal, deranged cult be allowed to function normally? Why normalize what's preposterous?

https://www.electproject.org/national-1789-present

https://en.wikipedia.org/wiki/Voter_turnout_in_United_States_presidential_elections