To be honest, I'd bet that RG3 played a big role in his continued presence in the game. I think he wanted to stay in- one of the interviews in the (Washington) Post quoted him as saying he was the best option, that's why he was starting. Which is probably one of the few times he'd made himself look like an idiot. Playing hurt only hurts your team.
I get that it was a big game and he wanted to play. But Cousins could have gotten something going if he'd been put in when RG3 re-hurt himself in the first quarter.
Bottom line: It was selfish of RG3 to insist on continuing to play when he was clearly no where near his full capacity. It was stupid of Shanahan to let him continue to play when he was clearly just hurting the team. It was just pure rude to Cousins to toss him in at the end and let him get smacked around. Unfortunately, thus is the nature of being a Skins fan, as Nyanko points out. Bad decisions all around.