If Evans took one more shot per game, he'd be in Greivis Vasquez range in terms of shots per game.
If Evans took two more shots per game, he'd be in Raymond Felton range.
He's already averaging more ppg than both of them.
Three more per game puts him in the Tony Parker/JR Smith/Paul George range. Parker has a higher %, but it's not like he's launching threes all game, either. Three more shots per game is less than one extra shot a quarter. It's not like he's all of a sudden going to go into Kobe mode by taking a few more shots per game. If they're open and in rhythm, why should he pass them up? He could easily get those extra shots by upping his minutes per game to an average of 33 per game, which seems to be OK for an aging, one-eyed Tony Parker. We're not even talking about Kobe minutes. Pre-Smart he was at about 37 minutes, which would put him right now at 18ppg-ish, the number you mentioned, which would put him at about 18-5-4-2.
Just imagine an offense build around him, instead of one build to spite him. He has the skills for 20ppg right now. He shot selection is better right now. His % isn't going to drop that precipitously with 3-4 more shots per game, (unless he decides to just launch from 3, which wouldn't make sense). He just needs the 4 more minutes per game, really.