A lot of these posts assume a static performance by QB's with no consideration for either growth or regression.
Some of them do stall when the get into the NFL, some regress for any number of reasons and a special few grow and never regress. My basic point is there is usually a constant change going on of some sort during a QB's time in the NFL.
Matt Moore once said that a backup QB is always only one play away from getting on the field and they need to be ready. I would extend his remark to point out that every QB on the field is only one play away from leaving the field.
I feel we should never underestimate the value of a competent backup QB. Every backup QB can be considered either a player who has reached his limits, but can play effectively, or a player that is undergoing growth. Where that growth leads to is tough to predict because coaches tend to favor experience on game day. This means a backup QB needs to wait for an opportunity to get on the field or demonstrate a lot of growth at his the next training camp.
All this is happening while new draftees are being brought in every few years "just for competitions sake".