Just a little history lesson for everyone on North Dakota if anyone cares.
The Big Sky actively tried to get every scholarship FCS school from the western US into their conference. They were pretty much successful. They had the idea in 2012 to expand and try to get North Dakota and South Dakota to join their league (and hoped to later add NDSU and SDSU). Their plan backfired when the MVFC added South Dakota. As a result North Dakota was basically on an island all alone in the Big Sky.
Skip ahead to last year. The Big Sky basically wanted North Dakota to leave and UND wanted to join the Summit League to rejoin the other Dakota schools. In fact the Big Sky even waived North Dakota's exit fee. That's how bad they wanted UND gone.
Actually I'm not sure if anyone is aware of this, but the Big Sky opened the door for New Mexico State to join their league. However NMSU doesn't want to join because they want to keep FBS football. Idaho who was already in the Big Sky for all sports except football took their offer and is dropping down to the FCS level (as a Big Sky member).
So at the end of the day, the Montana and Idaho schools will stay in the Big Sky and the Dakota schools will stay Summit/MVFC. Also a conference cannot just decide to be FBS. There are only two conference that do not currently sponsor FBS football that could sponsor FBS football. Those two conferences are the WAC and the MVC (not MVFC). So if the Dakota schools and Montana schools want to go FBS they would have to join up with New Mexico State do it through the WAC, not the Summit League.
Now let's look ahead to the future of the Summit League. Fort Wayne will most likely join the Horizon League at some point within the next few years. (If the MVC takes NKU then IPFW would probably be the top candidate to take their spot). There are rumors that UMKC might rejoin the Summit League next season. If the MVC takes Murray State I would have to think that Western Illinois could be looked at by the OVC. It would definitely be less travel for WIU and they would already have an instate rival in EIU. So worst case scenario I think the Summit will be down to 7 teams. Maybe they could get Chicago State to come back if they don't shut down.
With that said I think if the MVC invites SDSU, I think SDSU would definitely accept. It would have to be a no brainier for them. The Summit League could get a little better, but the stability of the league long term has to be a little bit of a concern. On the other hand there are plenty of schools (probably lots we wouldn't want either) out there that would jump all over an MVC invite. The Summit is pretty much stuck with whatever they can get right now.