ND going undefeated in 2018 and then getting blown out in the playoff game means that ND could/would get rejected and replaced by a Power 5 Championship loser if they went undefeated again with a weaker schedule. Sure Alabama can get away with scheduling blow-out teams - but life ain't fair. Non Power 5 ND has to beat top tier Power 5 teams in the regular season to get back into the playoffs.
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Though except for last year Alabama has proven many times it belongs