The people who have the state-sanctioned monopoly on violence are the ones who get to decide when a protest becomes a riot or unlawful assembly.
I’m not saying they’re always wrong but when only one group gets to pull the card that allows them to shut down protest it creates perverse incentives.
Practically, the media and the public at large. I forgot the source, but you can pretty much directly see the negative effect of e.g. bridge blocking on public sympathy for a cause.
But it’s trivial for police to incite violence. We saw it all the time in the US during BLM protests. A protest starts peacefully, then the protesters are pushed by riot control, then rubber bullets are fired into the peaceful protest, and now it’s not peaceful. Sometimes the protesters would even get surrounded and flanked so they can’t escape the descent into a riot.
If change does occur, what will happen is they will repaint history to make it seem as if the most cowish placid of the protests brought the change, to fool the public. And people actually believe it.