Here are some stats. Analyze away.
Joshua Nesbitt:
2008: 808 yds, 2 TD, 5 INT, passer rating 96.3
2009: 1701 yds, 10 TD, 5 INT, passer rating 148.7
Tevin Washington
2011: 1652 yds 11 TD, 8 INT, passer rating 155.4
2012: 1222 yds, 8 TD, 4 INT, passer rating 147.4
Vad Lee
2013: 1561 yds, 11 TD, 10 INT, passer rating 127.5
Justin Thomas
2014: 1719 yds , 18TD, 6 INT, passer rating 153.6
2015: 1339 yds, 13 TD, 8 INT, passer rating 119.1
2016: 1559 yds, 8 TD, 2 INT, passer rating 157.0
TaQuon Marshall
2017: 927 yds, 10 TD, 5 INT, passer rating 123.0
1) Marshall is pretty clearly the worst passing QB we've had. Our WRs are average, but I'd argue that this is by far the best group of pass-catching A-backs that we've had. Searcy and Lynch especially are reliable downfield threats if we can get the ball to them.
2) I think it's pretty obvious that QB performance has an extremely strong correlation with overall team success. I think it's also clear that our most successful teams pass the ball more. The 2 seasons where we passed for the most yards were 2009 and 2014. Not surprisingly, that is also when we had by far the best teams. IMO we need a 1800+ yard passer to have any kind of sustained success, and what we saw this year was not even close to that.