re: Auburn has no reason to exist.

What was the logic of picking USC over Clemson?

When the SEC decided to expand to 12 members in 1990, South Carolina was an independent in football & in the Metro Conference (remember that?) for basketball & non-revenue sports. The first expansion member - Arkansas - badly wanted out of the moribund Southwest Conference. To get to 12, the SEC first courted Florida State (which said no) & then Miami (which also said no). After those 2 snubs, South Carolina was the next best option available in terms of geography & level of commitment to big-time football. In terms of athletic accomplishment it added absolutely nothing, but it did expand the SEC's geographical footprint & it turned out to be a decent cultural fit.

The SEC, to its credit, has never tried to court programs that didn't already clearly want out of their old conference. Clemson in 1990 wasn't looking for a new home. Florida State's decision to join the ACC meant that its status as a major football conference was assured, so from that time onward there was no point in switching leagues. There was a time - from the late 1960s to the mid 1980s - when Clemson would have jumped at the chance to join the SEC, but those days are long gone, & for its part the SEC never asked. Clemson is, after all, a founding member of the ACC, & while that relationship has often seemed like a bad marriage in which the couple stays together for the sake of appearances but aren't even pretending to love each other, Clemson's administration had long ago concluded that there was more to lose than gain by switching leagues. They remembered what happened to USC basketball after it withdrew from the ACC in 1970 - it went from a championship-caliber program to complete irrelevance in about 6 years, & has never accomplished anything since then.