If it comes down to it UNC wouldn't hesitate to leave Duke behind. However UNC has all the leverage. If they want, they can request Duke to come along as a package. It would be like a 5 star recruit saying I'll only commit if you take my 1 star buddy too. Yes, you waste a roster spot on the 1 star but it is worth it.
Also Duke has a great brand. You don't think ESPN or FOX want that brand in their portfolio? Football is #1, #2, and #3-#28 but BBall does add value. It won't be totally ignored. Didn't we just see a BBall school tag along with USC?