It really depends on whether Sankey's goal is to raise more revenue by expanding the market map or if his ultimate goal is to build an NCAA-killer.
If it's the latter, then market matters less than name.
I think one of the big issues is if college football is going to continue being as strong they can't just abandon the west and the north. I think the move the SEC made was so big that it you can't stop there, it's just a matter of who does what. If the SEC is aggressive may be they can claim more territory. Fans still have regional allegiances, so just the south won't be enough to leave the NCAA or even to have just two mega conferences.
If not, the only thing keeping the Big 10 from taking over is their own arrogance (the AAU nonsense in particular). The Big 10 could just add Texas Tech and and Kansas, 6 teams from the Pac-12, and then Notre Dame, Clemson, FSU, North Carolina, Virginia and one more. Then they just make 4 7 team divisions. They'd be plucking teams from the top half of the ACC, Big 12, and Pac-12 so there wouldn't be a reduction in revenue. They'd also be encircling the SEC. Then it would be a matter of waiting out the SEC because eventually there would be no choice but to merge, given it would be a national product vs. a regional one. Not just that but politically, media coverage, etc... would be fawning over Notre Dame, Southern Cal, Ohio St, etc... like they always have.
In the long run I think it will be a matter of who is willing to be aggressive enough. The SEC made a real aggressive move, but it's certainly not a NCAA-killer yet. They can't surpass what we have now with just the south on board. I also don't think things can stay the way they are now, the SEC is way too top heavy and the three other conferences aren't just going to automatically vote all those SEC teams into a playoff.