I think you're giving the ACC too much credit while downplaying the Big Ten.
I can't argue with a suggestion the SEC is the elite conference (especially with adding UT and OU).
But, Ohio State and Michigan are two of the five most powerful athletic programs in the country. And Penn State is right behind (#2, #3, and #6 nationally in total athletic department revenue). In fact, the Big Ten has four teams (add in Wisconsin) before the top ACC school, #12 Florida State.
Top 25 athletic department revenue: ACC has three teams. Big Ten has nine.
The ACC would get swallowed up before the Big Ten. The only two conferences that are "safe" long-term are the SEC and Big Ten.
There's no clear advantage for Nebraska to leave for the SEC. And FWIW, Big Ten academics are far superior to the SEC. Nebraska is in a great position in the Big Ten.