I'd have to diagree with that one. Thinking back over the past couple of decades, very few guys who looked weak in their first year went on to have great careers.
Dunigan was an instantly successful second year starter (and he could not have been expected to displace Warren Moon as a rookie), and when Allen arrived the next year he challenged almost immediately for a starting role. Ham arrived a couple of years later and also looked great from the outset. Mopre recently, Ricky Ray needed very little time to shine.
In Calgary, Garcia and Dickenson both played great as soon as they got the chance. In Hamilton, the only two successful rookie QB's we've brought up in recent memory were Hobart & Kerrigan, both of whom had great results within months of setting foot in Canada. Everyone else has looked second rate from the beginning, and forever after.
Calvillo is the only guy who stands out as being very successful after struggling early, and I still say he had flashes of brilliance during his years here. McManus and Khari warmed a bench for years, but they both backed up some pretty good starters so likely had little chance at moving up the depth chart.
In contrast, I could name countless perennial backups whose reputations promised greatness, but they got cut loose before they ever really established themselves.
So to me, it seems that there is little upside to grooming a young QB for years if he doesn't look promising early on. Maybe I'm overlooking some examples that prove otherwise ... ?