Our team system still requires individual battles to be won. Better players are more likely to execute their assignment (do their job) against other teams better players. Stepping down a level in skill makes it that much harder to beat the Clemsons, UT's, Uga's, etc. There is a truth that has become cliche, but it's still true: football is a game of inches. If a guy is a hair slower, a hair less quick, a hair less decisive, a hair off target, etc... it matters.
And some of the backups may be just as skillful as the guys they are replacing, but the lack of experience hampers the full potential of that skill to be realized. You hear all the time guys who've been in the system a while say "Now I can play fast..." It's not that they were slow to begin with.