Imagine this scenario: Maybe 80% of the protesters are totally planning on going home at 8pm. You have 15% who are stubborn and don't like to be told what to do; they'll go home at 9pm if they're asked to insistently, just to show that you're not their boss. And 5% who are downright looking for a confrontation and won't go home until it's clear they won't get one.
What happens when you apply your rubric?
Well, when you apply your force at 8:30, those 15% move from the "stubborn" camp to the "confrontational" camp. In the 80%, there will be people who see the unnecessary violence, and move to the "stubborn" camp; in addition to people who tried to get home by 8:30 but couldn't for whatever reason.
Now you've got 30% of your protesters in the "confrontational" camp. The police get more defensive, and start doing stupid things like shooting people before 7:30. Congrats, now 40% of the people are in the confrontational camp.
Malcolm Gladwell recently posted a chapter from a book he wrote, concerning The Troubles in Northern Ireland, as a podcast recently; it addresses one of the core assumptions in your suggestion, that people are simple cost-benefit calculators, and so that with enough force, you can make impose your will on people. It never really turns out the way people think it will.