You don't need to believe that the current state of affairs is "fundamentally ok, or very close to it", quite to the contrary. What you need to believe is that the current state of affairs could be made much worse if one is not careful.

I think most people agree that through history we've made a slow climb up a mountain. And it's always easier to fall down, than it is to continue climbing. One could look down into the abyss and say "we must be careful not to trip", or one could look up at the top of the mountain and say "we must get there at any cost". In this metaphor I'd say the revolutionary would be looking at another peak in the mountain range and say "we must descend into the abyss if we want to make it there".