I''m of the opinion there are two ideal back-up QB's in the NFL today. There's the guys who are Hasselbacks or the like, once starters who have been in the NFL for many years. He's probably the best, but I mean the guys like McCown, Flynn etc. etc. They bring experience, they know their role, and they can help the starter from the bench.
Then there's the other class of back-up QB's. The class who are developmental QB's. You don't get any of the benefits of the veteran experience, but they have potential. They may be long-term starters down the road (not likely but who knows). TT and Aaron Rodgers are here. And guys like Mettenberger, Murray and McCarron were/are those guys.
EJ is neither, thus we should be looking to replace him. Because at the end of the day, if your starting QB goes down for an extended period of time, you're screwed unless you get insanely lucky.
There's also an argument that we are in a win-now stage and our goals are just to make the playoffs, thus trading for a McCown or Hoyer to hopefully go .500 or more if/when TT misses a game or 3 would be a decent move. They would be improvements over EJ for the short-term and we would lose some assets.
Either way, we can do better, and I hope we do.