If BSU joins the MWC, and the MWC gets a BCS autobid, then they'll have it for 4 years regardless of membership. If the MWC lost even 4 teams, which is a lot...they can just add 4 more teams from the WAC/CUSA (like Nevada, Fresno, Idaho, Utah St). Depending on who is left over, BSU could still find themselves in a good regional conference for them. Would they have an autobid in 5 years, maybe not. But they don't have 1 now in the WAC nor does it look like they ever will.
MWC if they lost Utah, BYU, TCU, New Mexico over next 4 years:
Fresno St., Nevada, UNLV, SDSU, Boise St., Idaho, Utah St., Wyoming, Colorado St., Air Force
WEST: Fresno St., SDSU, Nevada, UNLV, NMSU, Air Force
MOUNTAIN: Boise St., Idaho, Montana, Utah St., Wyoming, Colorado St.
I think the very fact that the desired WAC teams want to be in the MWC, the numbers are there for MWC to survive even if they lost as many as 4 teams.
The reality though, it that the number would more be 2 (Utah to Pac-10, BYU or TCU to B12...if B12 lost 2, then probably both go....but if B12 loses 3, I think you might see a CUSA school added as the 3rd). New Mexico or UNLV make sense for a 3rd option, but it pushes the footprint out west when after losing schools, the conference might look to refocus on lost areas.