All of them compete against each other in all the slots....all the time. This isn't some new feature at the advent of television. And its not like all B12 games have been isolated to single, non-competitive slots, even interconference. The only thing that is going to change for the remaining B12 teams is they'll get more T1 opportunities than today. For the past 5+ years, the bulk of the B12's T1 games have been Texas, Oklahoma, and whoever is playing them.
The top spots for college football are Prime, Noon and afternoon, in that order. The slots draw between 9-13m on average in total viewership, spread over the channels. How many watch is a combination of time, channel (T1 channels will always outperform T2, which outperforms T3, regardless of who is on), brand, competitiveness, and how many other games are on at that time. Alabama/Georgia won't draw 9m viewers on the SEC Network, but will on ABC, nor will they draw 9m viewers if Georgia beats Bama 54-0, especially if other games are on. People will flip around when games aren't nail biters. You need all to work together for the big numbers.
You can also kind of back into the numbers. If you assume being the apex of all five (Alone on prime time, end of season, 10-0 vs 10-0, playing for something big on the line, coming down to the last play to win it, between two big brands) is 10m viewers, then taking one out gets you to about 80% of that, take out two, 60%. The only one that is likely more than a fifth is channel, because you could put the two biggest brands on FS1 and it still won't pull 7.5m, not enough homes have it, people hate searching, etc. So with that in mind, here's how games break down:
8m+ = extremely rare, massive brands, with great records, in close games in best spots, top 1% of regular season games.
4m+ = elite content, probably 5-10% of all CFB games
3m+ = extremely good content, this and above is likely 20% of all games. Usually non-big brands having great games or a Globetrotter playing a General.
2m+ = You did the job if on a T1 channel. Not awesome, but not terrible, top 30%
1mish = disappointing T1 content, fucktabulicious T2 content, this and below is the bulk of all CFB games, regardless of conference.
500k-1m = standard T2 content
<500k = T3 content, fodder. Games between two bullshittery hapless teams who can't wait for the season to end to put them out of their misery.
Will the B12 still need to compete with the B10, SEC, and ACC for viewers, yes. Will the new B12 need to compete with the eyeballs of Texas/BlowU in the SEC, yes. Will they get 2 viewers while Oklahoma/Missouri gets 13m? Uh, no. The SEC will have more elite content then the new Big 12, no argument there, that's why they and the B1G are paid more, but that's only 3-4 games a week.
Depending on if the Buffs PR machine keeps this up, the B12 likely will only see a handful of games ever reach that high elite and rare level. Maybe 3-4 a year, tops, with one being the CCG. The B12 could easily live in the 2-4m range of T1 content and the 500k-1m range for T2 without Texas and Oklahoma. They'll still need to use this opportunity of more windows than before to do well, if they hope to grow their contract in 6 years, but they shouldn't see all their viewers evaporate.