I believe Drew mentioned at some point or another, or at least hinted at, that if McKeithen had lived then another Watergate-level scandal or something similar would occur (apparently McKeithen was known for utilizing dirty tricks of his own in Louisiana) destabilizing his administration and resulting in his resignation approximately round the same time Nixon resigned IOTL. Then Birch Bayh would probably lose re-election in 1976 to Reagan given that Reagan was so close in FLG to winning Hawaii (basically Florida 2000 but 24 years earlier) and beating Wallace. Then he loses re-election to Hugh Carey in 1980 due to the recession the late '70s and international upheaval. Then it starts to get murky. There was a non-canon update to Rumsfeldia after it ended wherein a user posited the idea of Rumsfeld losing in 1980 to Carey and returning to Illinois with his tail in between his legs. Carey would lose re-election in 1984 to liberal Republican Charles Mathias with Richard Lugar as VP. Then in 1988 Mathias declines to run again and Jimmy Carter is elected President in a landslide on a unity ticket between the Dems and the WTP with Pete McCloskey as his Vice President. The one-off ended with Carter taking the oath of office.
So, was Agnew pivotal in setting the stage for the Decade of Tears? His slithering into the Presidency had several knockoff effects.
1. He helped Donald Rumsfeld go from being another White House flunky to the top Presidential adviser, turning him from just another SOB into a psychotically ambitious ideologue.
2. His hardcore right-wing extremism and dogmatic foreign policy effectively laid the groundwork for the TTL rightward shift of the GOP and Rumsfeldia.
3. His horrible antics and corruption eroded faith in the political system more than Watergate and contributed to the breakdown of the two-party system.
4. His rushed impeachment angered right-wing forces and may have accelerated polarization faster than it did OTL.
5. His smashing of the economy through worse inflation and a worse oil crisis contributed to even more political radicalism and further eroded the Keynes consensus.
6. Slamming the door on China led to the rise of the Lesser Mao, who further destabilized America by unleashing an epidemic of heroin.
7. His TV Show on Hughes Network and willingness to become a proud ignoramus contributed to massive political polarization.
Without Agnew coming to power, would the Decade of Tears have been avoided entirely? TTL Newt Gingrich's alternate history has Agnew not reach the Presidency, preventing Rumsfeldia, which is treated in-universe as some right-wing power fantasy.