Yes, it's a good sized market but San Diegans are notoriously bad sports fans. San Diego is an incredible place but historically they haven't supported the local teams (but perhaps with such a good Padres roster that will change).
I agree that SDSU probably is already on or at least not far from the PAC 12 level. I just dont think SDSU will wind up adding enough to the PAC for the PAC to pull the trigger. That sounds like a PAC and SDSU problem though.