I don't know the MWC's contract but my guess is they have a GOR. It's pretty standard with most contracts and they have penalties in place if a school chooses to leave the conference before the contract expires. Do all the school want to agree to a much more punitive GOR? I think Fresno State is hoping they will receive a P5 bid at some point. They probably wouldn't support a change to the GOR, along with SDSU.
Of course SDSU may eventually leave but I don't think that's an existential problem for the MWC.