I'd say Cousins is a decently average QB that peaks top 10 sometimes. Jimmy is a below average QB that was heavily boosted to peak ever so slightly above average by an insane run game, offensive scheme, insane YAC options, and a top 5 defense to keep him in games. Jimmy is also a turnover machine and gets hurt if you so much as look at him sideways. Honestly, if Niners went any other way at QB 6 years ago they have 1 SB win if not 2... they should have moved on from him after his ACL and never looked back.
Cousins is my cutoff for acceptable starting QB. He's consistently average to above average, with a few flashes of brilliance. He won't win you more than 1-2 games a year by himself, but he also won't lose more than 1-2 games a year on his own. Jimmy G has been similar but a step lower for most of his career. Consistently average to slightly below average, with a few flashes of good. Absolute garbage so far this year.
I agree, also people don't realize how boosted Jimmy was... we are seeing the real Jimmy right now without the insane cast and scheme of the Niners to carry him.