In reality, it dates all the way back to the Kennedy-Nixon election in 1960, through Vietnam and Civil Rights to Watergate and Iran, but the REAL start of the decline was in 1981-1989, the Reagan "revolution"...
There is little doubt that the decoupling of compensation to labor output began in this period. There is NO DOUBT that the progressive tax system that had served to get us through the Depression without going to fascism, helped win WWII and then built the greatest upward mobility in human history from 1946-1960 was essentially killed off by Ronnie "VooDoo Economics" Raygun.
He did more to set the course of destruction than anyone...until the Shrub and his gang of thieves were annointed to the Presidency in 2001. That became the start of Raygun-omics on steroids - with Bush tax cuts eventually becoming the Trump tax cuts and the current trajectory of the economy and the national debt. Interest payments alone on the national debt have surpassed $1,000,000,000,000 - a TRILLION DOLLARS a year in interest payments alone. The cold hard truth is this - the United States WAS once a great industrial power that could manufacture the best products in the world and innovate new technologies as well as realize them in marketable products that made real impacts on lives - televisions, radio, autos, mobility, healthcare vaccines and treatments, productivity enhancing software options and more. That era is long dead and the only things the USA does now are create, market, sell and steal profits from debt (national, college loans, mortgages, and car payments) and military hardware.
Much of this is due to the slippery slope of idiocy that the Republican Party has been riding for the last 45 years. From Raygum to Read-My-Lips to Fool Me, Can't Get Fooled Again to Death Panels to They're Eating the Pets, its been a non-stop slip and slide into a real world Idiocracy. But we should never forget that it was Reagan that started it all, and just as the GOP sees FDR and the New Deal as the end of their preferred era - the Gilded Age, so it is with Reaganomics and Trickle Down.
The difference now is that after 40+ years of attacking and going after all elements of the New Deal, the GOP is accelerating the country into a divided future and a dystopian lurch towards fascism that FDR and the country rejected in 1933. These motherfuckers are back for a second bite at the apple and this time we are seemingly lacking an FDR-esque figure on the Democratic bench to ride to the front and save the republic once again.
I am reminded of the classic Genesis tune from the mid-1980s "Land of Confusion":
"Oh Superman where are you now? When everythings gone wrong somehow..."