No one has argued that the o-line has been constantly great for the past 19 games. They have had issues at times. But they haven't been the disaster that you're basing your opinion of Goff on. They have performed like a typical NFL o-line. Most teams have lines that are dealing with injuries and aren't always cohesive.
You offer things like "massively" injured among other flowery descriptions but you aren't actually defining anything. Leaping to the conclusion that virtually no one could have done better than Goff has done under similar circumstances. Of course, we'll never get to the point where similar circumstances are accepted because the Rams have supposedly experienced the worst conditions an o-line has ever endured and it's just too much to expect them to do anything but fail.
So, why haven't the 49ers collapsed with their o-line issues? They have not only had o-line injuries and issues, they've been without a decent WR corps, lost their star TE to injury, had a revolving door at RB, their FA WR pickup has been in and out of the lineup...but somehow, Garropalo hasn't dropped to the very bottom of the league in efficiency.
No one is arguing against the idea that a QB would struggle if their o-line did indeed have an off game or period of time where they couldn't function due to injuries and a lack of continuity. But as you have asked for examples of QB's that could perform under the ambiguous and mercurial defined o-line situation, what about the opposite? What other o-lines have had 19+ straight games of such supposed issues?
Especially ones that were led by a successful scheme and coaching.
Someone suggested Occam's razor. That it didn't make sense that Goff could have just forgotten how to play QB. But it's actually the other way around. Does it make sense that the coaches forgot how to coach, the o-line forgot how to play, the receivers forgot how to catch, the RB forgot how to run and the TE's forgot how to TE?
It seems much more plausible to me that Goff is a QB that can excel under a unique set of circumstances and that defenses figured out a way to marginalize those advantages so that when the pressure and responsibility of normal QB life in the NFL was thrust on Goff, he has regressed to what he was before. McVay conjured up some great tricks to get Goff going, Goff hasn't been able to show that he is a better than average starter when conditions aren't unique and ideal.
