"--It all changes a little. Maybe a lot. From what Joly and Combeferre said, the Paris they saw, where it all went right for our fight--not much changed, and then everything did."

It's not idle philosophy. "How long--if everything we do has an effect, any effect-- how long before the Paris we might sometimes reach isn't ours at all anymore? Before our information becomes useless? And how would we know?"

He's not being rhetorical, and he doesn't expect Enjolras to have an answer, either. But it's something to consider.