People could debate what the exact perfect amount of force is under an infinite set of conditions. At the end of the day, he was in their custody, he went unconscious, they didn't notice, and they didn't render aid immediately. That was their mistake. The rest is speculation and opinion.
I have been through quite a bit of police training. None of it established kneeling on someone's neck as a way of holding a cuffed person in place until EMTs arrived. If the officer believed the threat was so great as to warrant that response, you wouldn't think the officer would feel comfortable with his hands in his pockets.
