When schools change conferences, it typically is pairs (OU/Texas; USC/UCLA; Washington/Oregon). So, if UNC leaves to join the SEC or Big 10, it would be with another school. No doubt that UNC is the ACC's most coveted commodity, and pairing UNC with UVA makes the most sense because it delivers the heart of the Atlantic Coast region. The most common mistake made when selecting expansion targets is "this team is good in football". That is so far down the list in criteria. TCU could play in the college football playoff for 8 years in a row, and Texas would still be the most coveted program for expansion. UNC and UVA to the Big 10 would bother the SEC. UNC and UVA to the SEC would keep the Big 10 out of areas closest to the SEC. The Big 10 is never going to offer NC State or VT; those are secondary schools in those states; it was a battle to get the Big 10 to invite Oregon and Washington, and they delivered their entire northwest.
UNC and UVA are the last really coveted nuggets not in the SEC or Big 10.