You can't lay the 49ers’ numbers entirely at Kap's feet.  His supporting cast was atrocious. They basically lost the entire OL, and more than half the defence. If you compare Kap's numbers last year to his best year, not that much of a difference. His TD to INT ratio was 16:4, very good considering the mediocre receivers he had. 

Regarding Cutler and Hoyer, in which two consecutive years did they play in an NFC Championship?

Regarding Rivers, since week 12 of 2014, the Chargers are 9-30. In almost every single one of those losses, Rivers had a chance to lead his team down the field and win, and he flubbed it. He has good cumulative career stats, but he is a terrible clutch quarterback and getting worse every year.