The military and police are human, but they're also the main path towards control. If you treat them good, they'll treat you good, likely until they slip too far and are unable to back down without facing consequences.
It's mostly a good way to avoid situations like Cambodia's killing fields since that was also done by humans.
It'll result in a mess, but a mess is better than torture-to-death camps and famine.
But the words "critical mass" don't seem much more helpful than the definition of tyranny. The questionable boundaries apply, it's like a "you'll know it if you see it" thing.
The problem here is perception. Some people may "see" an outrage that causes them to act. While others don't. Jan 6 and George Floyd riots are two examples of people "seeing something" that caused them to act.
But if you are going up against the most well funded military in the world by some margin - well, whatever is seen had better motivate a LOT of people.