> that's because many people don't vote or take part in political advocacy
That does not seem correct based on the study. 100% popular support increases chances of a measure passing by 0%. If even a small percentage remains politically active, 100% population support should make some difference, and it doesn't.
Secondly, I believe it confuses cause and effect. At some level, people have cognizance that their efforts make no difference, so they don't bother.
Denying access to elections and legitimate election fraud seem worse. However, I feel this underestimates invisible power. It doesn't take overt violence or the threat thereof to thwart democracy. In American politics, money does the trick. We need lobbying outlawed, and to prevent the revolving door between government and big industry allowing for things like regulatory capture. Pointing to lack of overtly violent means used to thwart democracy proves nothing.
As for regular people determining policy, you might have a point. However, between regular people and ultra wealthy people making the laws for their own benefit, I'll take the flawed-from-ignorance laws of the common man over the flawed-by-greed laws of the elite. We should also perhaps try democracy before writing it off. We haven't gotten there yet.