You can't go by production.
It would be like comparing Aikman to Roger Staubach and coming to the conclusion Staubach was a piker because he didn't have the production Aikman did.
I can understand different eras, but that doesn't mean you discount the obvious production. Today's passers are being asked to pass more than ever and that gives you that much more information into how well they handle it. Goff was tasked with throwing the ball a lot, and he did it well, very well. That shouldn't ever be minimized or discounted.
It doesn't matter to me if those players above Goff can be replaced via free agency.
Why not? Because it should. And it should matter even more to the people charged with obtaining these players. The fact is that even if a team thinks that a player might be their franchise quarterback, they're not letting him go. And it's the fact that shoots holes in the other positions.
I think that you're trying to dismiss this fact because it's absolutely true, and it renders any other position moot.
What does matter to me is if the over-riding opinion is that Goff is a franchise-QB... if he is then you draft him. If not then you don't.
Simple.
It's never "simple". If it were, teams would never miss. But in a draft where there are question marks about everybody - and there are - if you have even a chance of hitting on a quarterback who can set your team up for the next 15 years, you jump on it and be thankful for the opportunity.