No leftist theory of which I’m aware seeks change (at least, not the bulk of its change) through the existing system. This would be absurd in that it is true that there will always be inequities inherent to capitalism. Take the most socially democratic state in the world, and they’re still relying on wage inequities, forced labor, and worse.
Theory is great, but we have to apply the theory to our real lives as best we can. As I said, how effective existing social democracies have been is debatable, but they are a working model of some, but not all, of the ideas and policies. Grassroot movements seek to radically change the system using the system. That's the modern progressive experiment. The only way to find out if it works is to do it. Like anything else I believe success is possible, but not guaranteed.