First is that Fouss eats up bad defenders. Playing these poor teams and they don't stand a chance from a size, strength, athleticism standpoint. The other thing is that these teams lack capable scoring bigs. So does it matter if Fouss or Keita is playing in front of them? I don't think it does as long as someone is (although Fouss' defense has not been great).
When teams and defenders get tougher I expect to see why Young is choosing Keita over Fouss to start (and to be honest who cares who starts it is more on who finishes). It also gives the 2nd lineup more scoring.
But hopefully Fouss continues to excel and his passing and defense continue to improve. From a scoring standpoint, he is as consistent as they come.