Two things here fearsome.
First, when Cousins was drafted he was considered to have a weak arm and ran damn near an OL time in the 40. This is part of that equation in why he went in the 4th round. If his arm strength had wowed teams he would have went higher. Fans can go back and play hindsight games, just like the Patriots fans do with Brady. But the draft is pretty fucking thorough as it pertains to physical tools and skill set. And that moment in time spoke via the draft. What they can't measure of course is give-a-fuck, heart, etc. And in the end I consider the fact that a guy like him, being a midrounder, was developed by McVay to be even more of an argument in his favor as a QB developer. But on top of that you have Goff, whose standing in the minds of "football people" of the time was so poor that many thought he would stand in the way of Kroenke hiring a good coach. Fortunately McVay was willing to work with him and develop him.
Secondly I don't profess to know what McVay is thinking. I don't lend a lot of credence to experts who act like they do either. My own approach to this game is to trust my eyes, which precludes me from that stuff. Doesn't mean I don't guess at times, I do as it pertains to predicting performance of coaches and stuff like that. But whatever McVay wants the truth is there are three ways of acquiring QBs in this league and in some windows of time one or more of those three are limited or closed in terms of options.
So let's say you are right. That McVay doesn't want to develop a QB. When Stafford decides to hang 'em up will there be a good FA option? Or will there be a QB trade option who is up to snuff? These things determine the path in that moment. I have no idea what any of that will look like, nor do I know whether he'll just bail out for media money. But if he does stick around, then he's gonna be developing a QB even if that's not his ideal path. And this is football my man.