Coincidentally, I wonder if future history books will someday show that the zenith of the US was actually sometime in the 1960's.
The article does an excellent job of discussing specific American spiritual and cultural failures that occurred in the early ‘60’s. But much of the general failure was political.
“I don’t know if it was the Johnson administration, the handling of the Viet Nam war, or what.”
In my opinion both had an enormous impact on the political shift that occurred in the ‘60’s. Johnson’s great society certainly seduced many into a welfare state of mind.
Perhaps more pointedly, commencing in 1964-65 the American left seized every opportunity to disrupt the U.S. effort to contain the spread of communism in Asia.
It blossomed on college campuses and gave us sit-ins, civil disobedience, North Viet Nam flags in dorm windows, Chairman Mao’s Little Red Book and misplaced adoration for Che Guevara just to name a few. The press and the education system were active enablers.
And the left is alive, growing and active.
Our enemy is U.S. imperialism, the enemy of all humankind, - Bill Ayers, Educational Secretary, SDS, 1969.
(Students for a Democratic Society - currently  active at several major colleges, including the U of Chicago.)
Yes, the mid 1960’s were a turning point for the US. From that point things went down hill, until Reagan. And today we are again on that descent.