That seems like a bad assumption. USC and UCLA didn’t draw masses of subscribers to the PAC-12 network, why would they come to the Big Ten network?
I’ve read a couple of articles now that say it’s not so much about market size anymore, but about branding. Teams like Notre dame, Michigan, USC have a measurable national following and will pull subscribers across time zones, and that outweighs the benefit of a lot of local markets. That factors into why UO/UW aren’t worth it at the moment - they are primarily local/regional draws and don’t bring enough national subscribers or advertisers.
The PAC-12 is probably going to have to lower their standards to survive, unless we can wrangle a merger with the Big 12.
Limiting to R1 schools is probably the standard that has to go, because there just aren’t enough of them on the west coast who are also even close to competitive athletically. Nevada is probably closest. UNLV, New Mexico, Utah state, and Colorado state are also out there, but the more of them we add the more we guarantee relegation to mid-major status. Besides, none of them bring any market or branding.
SDSU is headed toward R1, is relatively competitive, and maintains a presence in SoCal, so if some sort of combo with the Big 12 doesn’t happen, SDSU seems like the closest to a no-brainer. UNLV brings Vegas, but that’s not really much. They have no national value and usually aren’t even competitive as a mid-major.
BSU is barely even R2 and is nowhere close to the Pac-12 level academically. They have 15 years of competitive history in football that had some national interest at its peak, but I don’t know that they’d pull subscribers even back then. Since Petersen left, I think that interest has waned.