The problem I have with your point is that while yes, starters usually don't lose their job to injury, the fact remains that Griffin really didn't EARN the starting job in any meaningful capacity.
Did Griffin at any time in the 2014 season or this offseason really SHOW that he was the best quarterback?
All we have is a forced Jay Gruden say in February that Griffin was going to be the starter and that was before most of the offseason work had begun.
It was clearly a move by ownership, along with the option exercise, to boost Griffin's confidence and hope that he came into camp re-energized and ready to make a difference from Day 1.
But you can't fool these players out there.
As Doc Walker and others have said, the players see who knows the offense, who is really committed to the TEAM and to his teammates to get better and allow them to get better.
Right or wrong, it appears that Griffin has about as much popularity on this team now that Jay Schroeder had late in the 1987 season.
Team captains evidently went to Gibbs and told him the team had lost faith in Schroeder and they didn't feel confident going into the playoffs with him as the starting quarterback.Other than two snaps in the Super Bowl while Doug Williams was getting his knee looked at, Schroeder never was under center for the Redskins again and was traded to Oakland.