I think people seriously undervalue the relationships in the QB room and the value of having a guy with experience starting, seeing lots of different coverages, blitzes ect in live action. There are plenty of examples of guys having tons of starting experience backing up established starters. David Carr has two Super Bowl rings for example backing Eli for years and years
The Chiefs have had the following guys as their backups in recent years
Carson Wentz
Blaine Gabbert
Chad Henne
Matt Moore
See a pattern? All of those guys we would be worried about if they had to play. All were failed starters. Hell Wentz has gotten several coaches fired because hes been so bad