I'd be interested to hear from older fans if these two conferences were likely "finalists" in such a realignment or if this only became clear later. 2001 basketball was really the first season I even vaguely remember of NCAA sports and by the time I was old enough to really start getting into it passionately (circa 2005, conveniently enough!!), I feel that there was already a sense that the Big Ten was the "richest league" with the biggest fan bases. I cannot remember what I thought about the SEC other than they were really good at football.
However, if you had asked me "back in the day," I suspect I would have perceived the Pac Ten as kind of uniquely secure given their TV markets, good academics (at the top anyway), pretty storied sports programs (again, at the top anyway!) and - perhaps most importantly - perceived geographic isolation from the other major conferences in such a way that might cause them all to rely on each other and stick together. And yet here we are!!
P.S. This just reminds me of how important it is to have Illinoisans with ZERO connection to U of I grow up Illini fans. Schools like Oregon State/Washington State in the Pac-12, NC State/Wake Forest in the ACC and Kansas State/Iowa State in the Big XII are pretty unanimously screwed if/when their conference falls apart because their fan bases are mostly restricted to people with direct connections to the schools. In other words, unless you grew up within 50 miles of Ames or you are an alumni of Iowa State, you probably grew up a Hawkeyes fan. Same for Oregon State vs. Oregon, Washington State vs. Washington, NC State vs. UNC, etc. Illinois is lucky first and foremost that we are ALREADY in the Big Ten, but we are also lucky that there are tons of people in Downstate and a decent number in Chicagoland who either didn't go to college or went to a small school without football who defer to the Illini as the state's team. We have a long way to go to inch closer to the instate loyalty of, say, Ohio or Pennsylvania (we will never replicate the dynamics in Iowa, Nebraska or even Wisconsin for obvious reasons, but that is fine!), but we are extremely fortunate to be a state flagship in a big state with a massive alumni base in the current environment, JMHO.