There are 134 D1 FBS teams. Only counting the 85 scholarship players, that puts us at 11,390.
There are 352 D1 basketball teams. With 15 on their roster (walk on and scholarship), we get to 5,280. So, about half.
Your point about Notre Dame and NIU is irrelevant. I’m not arguing that a bad team can never beat a good team. I’m saying that bad teams should not get a shot in the playoff because they beat a bunch of teams worse than the bad team.
If they show that they’re potentially a good team by winning all (or almost all) their games, great. But the point of the playoff is to figure out who the best team is. We already know these G5 champs aren’t.