I have used a 11k RPM grinder, not that I would have called it small the thing was like 10 pounds. So, I can can see how they might be harder control with newer motors and swapping steal for plastic.
Backing up, being safe with power tools is IMO more a mindset than a set of safety equipment. As such you need some understanding of the forces involved. Not, just to be safe right then but so you don't get lulled into a false concept of safety because the overblown worst case you picture did not happen.
You have absolutely no idea what an unbalance of even a few grams will do at those rotational speeds. The thing will escape your grip even if you're quite strong if you're not holding it very tightly.
I don't think anyone was seriously considering trying to hold it. My comment was more in line with "The iphone will desintegrate due to the forces involved and the shrapnel will cause serious injur". Granted, those disks can shatter and I assume an iPhone may also shatter, but saying it will is a different.
PS: I am going to leave this alone. It's dangerous, but I do hope someone posts a video of this at some point.
I have a hard time envisioning the kind of hold-down that would be required to hold a smooth iphone onto an angle grinder. That's going to be a really neat bit of engineering. And then of course the guts of the iphone will die anyway, I seriously doubt that it was designed with that kind of abuse in mind. But we can place bets on how many seconds it will last.
Backing up, being safe with power tools is IMO more a mindset than a set of safety equipment. As such you need some understanding of the forces involved. Not, just to be safe right then but so you don't get lulled into a false concept of safety because the overblown worst case you picture did not happen.