ND has made it very clear it will not join a conference for Football as long as they have a legitimate way to the CFP.
As bad as the NIU loss is, if they were to go undefeated the rest of the way they would definitely make the CFP. Even a 2 loss ND is probably a 50/50 bet for the CFP.
This is not one of their stronger schedule years, but they still play 8 P4 teams and no FCS opponents (which they never play).
They also have an agreement with the ACC that if they were to join a conference for football it has to be the ACC. That of course could always be negotiated, but they would have to likely pay money to the ACC to go elsewhere. They are a party to the ACC GoR in all sports but Football. General rule of thumb is that media contracts are 80% for football, 20% for basketball.
The best case scenario for the ACC would of course be ND joining ACC in football. Likely wouldn't completely close the future gap, but it will get it closer and would likely put significant distance between the ACC and B12 in terms of money.
Also, keep in mind that all this talk about money gap has almost nothing to do with where things are right now and alot to do with where they will be in 5 years.
For FY 2023 ACC paid an avg of over $44M per member, SEC paid out an avg of $51M per member.
Money differences between the schools in the conferences are NOT primarily driven by the media contracts. They are driven by the difference in alumni/fanbase size. The B1G and SEC are made up primarily of large, state schools that have alumni and fanbase sizes much larger than most of the schools in the B12 and ACC.
How 'good' a schools football program is a secondary (maybe even a tertiary) consideration for the media contracts. The primary consideration is how many fans you have and what media markets you bring to the table (especially in terms of the B1G, SEC, and ACC who all have networks where the partner gets to charge a higher carraige rate if you have a school in a market).