You can pose a concrete threat to stability via peaceful methods, in my opinion - there are plenty of historic examples. Violent protests in the modern world only create the narrative in the minds of the establishment that the opinions of those protesting are invalid and must be stamped out.
So I don't know much about protesting, but I was talking to someone the other day and they told me an interesting little anecdote. There was a city that passed some very strong anti-vagrancy laws that had upset a lot of people. To protest, they decided to occupy the city park after dark, one of the things that became outlawed because of said laws. The police would come to talk to them and said they're allowed to stay and protest, but they have to fine them. The protesters accepted their fines, stayed to protest, and on this went for about a week or so. To make a long story short, everyone kind of won in this. The protesters got to protest, the city collected revenue in both fines as well as court fees to have the protester's records dismissed. Everyone got to go home happy. Except the homeless, cause they didn't have homes to go home to and the protesters didn't change a thing in regards to the law. That said, I don't know how I feel about violent protests either. What good is the world to anybody if all we ever do is threaten to burn it down?
I think there must be a clear-cut campaign that has to be waged when it comes to protesting - simply showing up with a sign isn't enough, I don't think. I'm thinking Gandhi style rallies and hitting the economic system in a profound way. I must admit that the failure of the Occupy movement and the movement in Hong Kong really shook my faith in peaceful protesting, but at the same time I really do think that with enough of a coherent vision it would be possible to make change happen. But maybe I'm being naive.
>there are plenty of historic examples There are more examples of peaceful protests that fail to achieve anything. The most famous "peaceful" protest movements (MLK and Gandhi) were backed up by the implicit threat of violence by aligned groups. If you're interested in this subject, I highly recommend the book How Nonviolence Protects the State.
And yet there are many examples (probably more, in fact) of armed and violent protests also failing to achieve their goals, or having their goals distorted. It is true that the peaceful movements of Gandhi and MLK in particular were also accompanied by an implied violence from allied groups, but at the same time there were movements (especially in India) that had tried to achieve independence via violence for well over 70-80 years, always failing.