Well, Tom is the GOAT, not a backup, so not sure what the argument is.
Any backup will be a major step down from Aaron Rodgers and won't be able to come close to doing what he does. It's more about running the system he runs not making the plays he does, because no backup can make those plays. Basically, the idea is run the same offense for hopefully only a couple of weeks and don't make many mistakes and maybe a couple of lucky plays and good defense allows you to steal a game or two. Bottom line is if Rodgers is out for a long period of time , the Packers are done, no matter who the quarterback is.
Maybe you steal a game or two against a bad team while waiting for Rodgers, but, long term or playoffs it doesn't matter, the Packers know they can't do it without him.