I think the decision to let Ohio State play was correct. It wasn't their fault that Maryland, Illinois and Michigan all cancelled due to COVID. Those would almost certainly have been reasonably easy Ohio State wins. The Buckeyes were clearly the best team in the Big Ten this year. They beat Indiana by seven and Northwestern by 12; no one else was within two touchdowns. Forcing them to sit out the playoffs because of other teams' illness problems would have been a pretty bum thing to do.
Texas A&M, not Notre Dame, should have been the fourth playoff team.