I agree ... somewhat.
We can't use TB. Aaron, or Love as examples. TB is in a class by himself, even way back then. Aaron sat behind Favre, and Love sat behind Aaron.
How many QB's sat behind high level QB's and, when it was their turn, didn't pan out?
No offense, but I'm not going to do the research on that one, right now. It would take forever, and "didn't pan out" is too fluid a metric. For example, and just for example, did Scott Mitchell "pan out" after being drafted by the Dolphins in the 4th round?
So let's just keep this in New England, for a moment, and let's start with Bledsoe, who was the first big move of the "modern" Patriots, and let's not count injury/other replacement type games, or garbage time play, as starting year one:
Out of that group, the players who made it at any real level, at all, are:
Bledsoe - played right away
Brady - sat his rookie year
Cassell - sat his rookie year
JAG - sat his rookie year
Brissett - sat his rookie year
Jones - played right away, flamed out when SHTF
Hoyer (UDFA) sat his rookie year
So, just looking at recent Patriots history, we see that sitting, and learning as a backup, is not any sort of problem.