For an organization wishing to preserve its specific mission, I wonder if one positive step would be clearly defining the point at which the organization would say "mission accomplished" and pack itself up. Riffing on your example, suppose an anti-ad organization said, "We want to achieve a goal of 20/50 states banning highway ads. Once we get there, our organization will be wound down [in some way.]"

There are reasons that might work badly, though. Without the org continuing to generate political pressure, public awareness, or money, the originally-achieved goal state could backslide. But ... change is inevitable. Perhaps at that point you try and get the org back together,; if you can, you can, otherwise you accept the world has moved on.

Easier said than done.