While I agree with your basic premise, that 3.5% "rule" is much more of an observed effect than an actual rule.

There needs to be an actual mechanism for the protests to bring about the fall of the authoritarian regime. Unfortunately, in our current context, a lot of the feedback mechanisms that should cause protests to change actual policy and affect the people in power are broken, largely due to the Republicans' efforts over the last several decades to eliminate accountability both from the actual institutions and as a valid concept in our national consciousness.