Clearly Case has improved and the game has slowed down for him. You can see that in how his completion percentage has improved over time.
Look - Case was thrown to the wolves a bit in 2013. Arien Foster was hurt. The starting TE was hurt. A couple of defensive players were lost for the season.
Hell, Kubiak had a stroke at halftime of what was brewing to be an upset of the Colts, and Wade had to take over. Sure - Case went 0-8 that year, but I don’t think Troy Aikman could have done much better than 2-6 under those circumstances.
Case Keemum is now 21-10 as a starter since 2014. That’s 67%.
Here are the other QBs who have won 67% of their starts since 2014 (minimum 10 starts):
Tom Brady 81%
Tony Romo 76% - retired
Peyton Manning 76% - retired
Drew Stanton 69%
Ben Roethlisberger 68%