That's an awesome animated gif accompanying your post.
That could very well happen. But I still would be against it. For two reasons:
1) I am not sold on Goff. Either way. I really don't know if he will be good or bad. Last year was pretty much the "perfect storm" of crap that he was dealt his rookie year (bad offensive scheme, bad offensive line, bad receivers, 1st year in a pro-style system). I think the trend is going upward. Will it be enough for him to progress to become a franchise QB someday? Have no idea. But still I think he plays the whole year.
2) I just don't think he and/or the staff think much of Mannion. He has always performed pretty well in preseason games. But my guess is they don't like the way he locks onto primary receivers and probably aren't thrilled with his delivery. I keep hearing they are pretty much pegging him as a backup at best. So I am speculating that they don't feel they'd gain much by benching Goff for him.
Of course though if Goff falls off a cliff (or your high limb you are on), then all bets be off.
Joe C.