You're very wrong on this. Nixon won the election in 68, He also WON the deep south. Nixon also won in 72 by a very large margin. The GOP probably would have won in 76 if not for the fallout from Watergate which tanked the GOP. But they won in 80, 84, 88. From 1968 through 1992, they only lost 1 Presidential election and the Southern Strategy had a large role to play in that by reducing Democrat strength in the South.
The big issue with the Southern Strategy wasn't just with the politicians. It was with the messaging that the party embraced. The campaigning and the message that they, and other Republicans, utilized drove the wedge between the party and the black community. Black America remembers the painting of black women as welfare queens and all of the other dog whistling politics the GOP employed over those years. Not just at the Presidential level but also in state politics and Congressional politics.