Dana White nailed exactly the reason that the stoppage was absolutely fine following the event:
"I agree 100 percent with that stoppage," he said. "He was on crazy legs before the kick to the head happened. And again, there's a fine line between stopped too early and stopped too late, and I'll take stopped too early every time. I thought he was hurt before the kick to the face."
In MMA we don't have a ten count like in boxing. MMA fans talk about the ten count far too often as a huge safety issue but when it is used correctly, the idea is to make sure a fighter is fit to continue before restarting the fight. MMA referees do not have that luxury, they're forced to make immediate calls in the interest of the fighter's safety.
Brenneman's legs were rubber when he stood up just before the kick, he appeared to be using the cage to hold himself up and then got kicked in the face and went down.
Without a tool like a ten count, Mario Yamasaki can't check on Brenneman and make sure his eyes are clear and he is adequately responsive. He has to make a decision in that second and he saw a hurt fighter take a kick to the face and go down.
Almost any referee who see a fighter with his legs gone using the fence or the cage to stay up are ready for any flurry of strikes or single big shot to stop it.
It feels like a phrase I don't say very often, but Yamasaki's call was completely reasonable and correct.