You are Captain WIN, because you always have the right answer.
Cousins hasn't been as efficient as elite QBs, but his QB rating was still in the high 90s over the past 3 seasons on a team which hasn't had a consistently good QB in decades, including a 5,000 yard season. A QB like him hitting FA is extremely rare, which is why teams were lining up to throw money at him, and Keenum had far fewer takers.
Keenum is a game manager. If he can consistently put up those efficiency numbers, then a team could certainly do much worse. But as we have seen with T. Taylor and Alex Smith, when a team has a QB who is efficient but unable to generate much yardage, teams tend to long for more.
I view Cousins the same way the I view Stafford. He does not scare you on an average team, but I certainly do not want to see him join the Vikings. Didn't he have nearly 400 yards and a 145 QB rating against us in 2016? And now he's on a better team..