Right, one way to compare would be to compare the previous Thursday's game's rating since that was on an over the air and then you would have a plausible ratio of what to expect for a college football games ratings.
The Bills Rams game (which wasn't competitive) drew 21.3 million. So I think for the very best college football games Amazon could get they could expect something like 2 million viewers for a primetime game
It'll be interesting to see what tonight's dog of a game gets in ratings.
It probably depends a lot on demographics, specifically age.
For people under 35, if a game was exclusively on Prime or something they could subscribe to for $5/10 month they'd probably be more likely to see it than if it required a cable subscription or OTA (lots of younger people don't even know about OTA, even some middle aged people who grew up with cable barely know about OTA).
For the over 35 demographic having it streaming only probably plunges the viewership.
I'm just pulling 35 out of my @$$ but there's definitely an age where streaming makes it more likely to be seen by younger audiences. The way to really maximize viewership in the right now would be if ESPN had their cable tv games also available to stream on ESPN+ for a low monthly no contract fee. That's not in their best interest right now but some day that will probably happen for a few years when cable TV actually is dying.