ehh... faulty logic. Stafford's only thrown for 5k yards once and they won ten games and made the playoffs that year. they've also made the playoffs 2 of the last 3 years and last year obviously without megatron, and with a depleted backfield.
on top of that, there were 12 qb's with losing records last year, and 2 that went 8-8 and only 2 of them threw for over 4k yards (carson palmer and Andy Dalton)
So realistically there's not many qbs at all racking up a ton of yards with losing records, and Stafford certainly isn't one of them. He definitely passes the eye test to me, and yes I could see him winning a superbowl if he weren't playing for the Lions.