The right comp for Bane is Klay Thompson. His performance the past couple years tracks real close to Klay at the same age, without the all-NBA accolades.
It's actually a bit unfair to Bane, as while Klay was a better pure 3 shooter, Bane is much better at putting the ball on the floor and driving to the rim. He probably won't be as limited offensively as Klay (though Klay's one dimension is pretty unreal).
If you want to nitpick, sure, Memphis could have waited another year to max him, and if he falls off a cliff they'll look silly. But no, this is not an unreasonable contract for a player of his age and caliber.
As incomprehensibly flawed as it is to evaluate a player in terms of ppg, yes, Draymond is the most offensively limited player on a contract of that size. A player with Draymond's offensive limitations and average defense would be out of the league.
However, Draymond is one of the best defenders in the league. If he was even an average player on offense he'd probably command a max contract.
So yes, a player being paid for one skill can look really bad when evaluated in terms of another skill.