Don't think they'll leave the ACC. ND likes being indy, but they also like the perks of being an unofficial football member of a conference (bowl tie-ins). Plus, ND has to worry about their other non-revenue sports and I think they fancy themselves bigger than Xavier, Butler, Dayton, etc. Nothing against those schools, but I'd fall out of my chair if ND agreed to be in the same conference as Butler.
ND fancies themselves bigger than everybody. I think you are probably right and I think they will eventually have to join the ACC in football also. I would find it hard to believe that they will get such a favorable deal when the 4 superconference deal is implemented.