Tiny TCU/ISU last year drew 4.4m opposite the Iron Bowl, which drew 6.6m. It was bigger than Texas A&M/LSU, which played in prime on the same day, and bigger than Aggies' games against Miami, Arkansas, and Florida, and only behind Aggy's game with Alabama. Texas A&M/Auburn didn't even get off the SECN. The B12's ceiling is no where near what the top of the SEC brings, but they still capture a lot of eyeballs in a tier below "big brands playing great". This is the reason the big money is coming into the SEC, as there are enough big brands at 16 that someone will always be winning, allowing that only the best games will be broadcast in the top two windows per Saturday. If you're not winning though, you'll will be swept under the rug from an exposure standpoint.
I mentioned before that this move is going to help Texas' audiences considerably. That doesn't mean this tide rises all boats. A LOT of games Oklahoma currently sees broadcast due to the B12's contract, where they are always choice 1a or 1b in any given week, will drop networks and many will be on the SECN as they'll fall to choice 3, 4, or 5 in any given day. That's going to be a major change. Over time they will likely see less marquee game times unless they continue to be 10 and 11 win world beaters vying for a title. There will also be days when Oklahoma State has a better draw than an Oklahoma game, not because Oklahoma State is a better brand, but because the B12 is guaranteed good spots and Oklahoma has to be louder than Texas, Alabama, LSU, and A&M, to get the best SEC weekly draws, and that is just in the western half! Oklahoma State just needs to be louder than the rest of the Big 12 on that day.
Those Thursday and Friday prime games aren't as good as ABC at noon/330/prime, but they're better than any ESPN2/U/FS1/Plus slot, so their 3rd-5th choices end up there, depending on what Fox does, which isn't a bad decision for them all things considered. That being said, the teams doing well in the B12 should be able to hold their own in most marquee slots, though wont be able to touch the top numbers Texas draws.