One factor I would weigh pretty heavily here: the amount of time Wagner and Hayes had to consider their actions.
I agree that what Wagner did was dirty as hell. Were I Adam Silver, I'd suspend him for a couple of games (EDIT: I got annoyed as I thought through everything while I wrote this post, and handed down a stiffer punishment at the end). Can't have players hip checked into the first row (imagine the shit-show if Hayes was sent flying into fans instead of the Detroit bench?). But Wagner had maybe a second to look over his shoulder, see someone coming, and decide to be a complete asshole. Not really enough time to think through the ramifications. Indeed, when he sees what happens to Hayes, he stops right in front of the Detroit bench (not the smartest move, right?) and starts to raise his left hand -- his body language suggests that maybe he's trying to show some contrition. It's cut off abruptly, though, when Diallo hits him in the back.
Hayes, meanwhile, flies into his teammates. He's pissed. He's scrambling to get up, presumably intent on retaliating. A coach is trying to hold him back. Hayes pushes past him, sees Wagner turned the other way. Instead of understanding that a punch under these circumstances would be an unforgivable cheap shot, he pulls back his arm and hits Wagner in the neck/back of the head with his forearm. Wagner is out cold on his feet.
Several seconds, several choices. Anger undoubtedly clouded Hayes' judgment, but he had multiple opportunities to deescalate. He chose not to.
I used to run an upperclassmen boys dorm at a boarding school, and when kids screwed up (all the damn time), I talked with them about the one-shot bad choice, and the clusterfucks where a whole series of bad decisions were made, moments when they could have done the right thing but didn't.
I'm draconian when it comes to shit like this, so I'd suspend Wagner for eight games, Hayes for twenty. Fuck cheap shots. You want to play dirty? Take a seat, lose your paycheck, and think about what you've done.