The article does bring up a good point: even the incredibly stable Ivy League considered expansion at one point (probably more.) Northwestern with its unionization (and the B1G's response to unionizing) may be a good fit with the Ivy League. In the end, if they do get kicked out of the B1G, I can see them joining Notre Dame as #15/#16 in the ACC or maybe even join the XII. Adding Northwestern and Cincinnati would be 2 big market schools adjoining the XII's two islands of Iowa St and West Virginia. I don't think Army or Navy would go down with them to FCS, though.
Northwestern was one of the two BIG schools the ACC was 'showing interest in' as disclosed during the Maryland-ACC lawsuit activity last year.
A number of years back before the old BE broke, Nothwestern even showed slight interest in chatting with the BE when the conference indicated it was considering adding.
Northwestern is the B1G's one private school, and know that means added challenges in remaining competitive in the conference. It may no be so different for Vanderbilt in the SEC.
The rules and obstacles would not be that much different from the B1G if Northwestern was in the ACC (B12 would be too unrealistic).
The ACC offers a menu of private schools: Wake, Duke, BC, 'Cuse, UMFL, Pitt as state-related; but there's also fb-oriented state schools such as FSU, Clemson, VPI, Louisville, GT, NCSU; and bb is pretty competitive up and down the league most years. Going there for Northwestern would not necessarily provide a greater abundance of friendly/compatible competition; and Northwestern would be giving up being a centralized geographic entity in the B1G along with their long history in the conference.
Northwestern would be foolish to leave the B1G unless it became a situation they could not afford it. And 'affording' would become an ACC factor as well.
Northwestern could downgrade if they saw it as a financial must. While hypothetically plausible, NW would not be an ideal Ivy League fit though. To start, NW would be a geographic outlier to the IL.