If Bradford doesn't produce and Davis takes over, then clearly Bradford isn't safe. The switch will have been made because the coach called for it. If Davis does markedly better after that, then Bradford will find himself on the outside looking in. Which is how you'd expect it to go with just about any team, really. I don't think Bradford has the job by default because of his salary though. That's not how Fisher is wired. Bradford has the job because he's simply the best guy FOR the job.
That said, this is all moot. Bradford is going to break out in a big, big way this year and there will be no further discussion about his longevity on this team. Demoff has expressed his desire to rework Bradford's contract with an extension, so they're already thinking long-term. Sam will have to either get severely injured, or completely screw the pooch this year to mess any of this up. I can't guarantee he won't get injured, but there is no way he's going to become a bad QB.