A couple of backups that are still on the Steelers roster make the list
One of the hot topics at quarterback over the past couple of years has been Mason Rudolph. He is still on the team after re-signing this past offseason, despite many thinking that he will not have any chance of returning. Rudolph went to free agency and was not able to find a better situation, so he returned for another year of being in a backup role.
Perhaps his biggest letdown to fans was their high hopes of him potentially being Ben Roethlisberger's replacement. That never materialized and Rudolph has led some stale offensive attacks when he does get a start or two. He has been a reliable third-string guy, even a backup, but he has never shown enough to warrant a top job in the NFL.
Expectations for a player could almost sink a career before they even play a snap in the NFL. Mitch Trubisky was drafted high by the Chicago Bears, but he was a below-average starter during his time there. He has now become a backup in the NFL and joined the Steelers last offseason to compete for their starting job.
He did win the job last year, but he was not able to make much of an impact in an offense that struggled. Trubisky re-signed this past offseason, and that is not the worst-case scenario that could have happened. He is not going to wow anyone if he needs to start a couple of games here or there, but you can do far worse for a backup.