They already did that for many years ... before the BCS became a thing.Other than Australian Rules Football, I can think of no sport that does it that way.
But the question for college football, is, of course, why have playoffs at all? Play the greatest and most meaningful regular season in all of sports, have a bowl game, and pick a champion, if you must, by polls, understanding that many dozens of teams will have “good seasons” by whatever measure they have. Rather than a grinding ESPN tournament that produces 1 winner and 132 losers.