Newton isn't just a bridge-type QB, he is a legit franchise QB. Allowing him to leave would be really stupid by Carolina, but then again, firing Rivera was dumb too...
His situation is somewhat similar to when Drew Brees was replaced in SD with Philip Rivers due to his shoulder injurey. The Dolphins chose not to sign him because they didn't know how the injury would affect his play. While Newton isn't as great as Brees, Newton definitely can still be a franchise QB.
As for backups:
Landry Jones was and is horrible. With him we'd be 4-11 or something like that, at best.
The best backups usually are former starters, guys like Teddy Bridgewater, Blake Bortles, Tyrod Taylor and Matt Schaub (even though on him there is very little playing time for several years, so hard to judge how he would do if he needed to start).
Maybe we could sign Mariota, Dalton or Eli Manning, all of these would be top-notch backups.
Rivers and Fitzpatrick probably won't be backups, in the case of Fitzpatrick even if he's signed as backup he always ends up playing. Fitzpatrick's only two seasons where he wasn't backup at any point in the season were 2011 and 2012 with the Bills, yet in every single season since 2008 he has startet at least 3 games.
So, in conclusion: While I would love to see us signing Cam Newton, I don't think that will happen since Ben already said he's returning. So getting Mariota or Dalton is the best we can hope for. But probably even that won't happen because the Front office doesn't seem to value the backup QB position. We never really had any good backup QBs in the Tomlin era. Just rookies or players who played very little, or old QBs not capable of much anymore, like Vick, Batch, Leftwich...