I don't thing that population is the best metric to use when determining franchise viability.
Just another small note, not sure if it means anything, but...
Working in a corporate office of a franchisor some of the offices in smaller cities are viewed just as important as our larger cities. Presence in smaller cities is important to the brand of our company.
That being said, I'm not sure that there are many potentially profitable markets for the NFL. Is it in the best interest of the NFL to move teams to different cities other than LA?