I'm not sure I'd call Enjolras naïve at all. Narrow, certainly, and therefore without much interest in, ah, the finer aspects of life outside his laserlike focus on the Revolution. (Poetry! I was talking about poetry, you pervs!) But he seems to be the one most focused on the French revolutionary tradition, which was a mixed bag of triumph and defeat, and by the end of 1830 it had already been stolen and co-opted twice by monarchs and dictators. It had also, mind you, survived a good ten years with a civil war going on and literally every other country in Europe allied against it, which was the actual context for the ruthless clashes of ideals that emerged on all sides in the crucible of 1793. And that's something Enjolras would be familiar with: how and why such things happened, and the full moral weight of bloodshed--even necessary bloodshed--in the name of principle. He's the one least inclined to let himself off the hook for executing Le Cabuc; he literally condemns himself to death for it, and follows through. (Another strike against the idea of Enjolras as naïve: he's the most warlike of any of them. It's easy to picture Combeferre as a statesman in a French Republic, but Enjolras? He's a general, a soldier-priest. Combat and revolution are his element, and he fights in order to cede the future to men like Combeferre.) As for whether the people would rise up... again, long revolutionary tradition. They rose in 1830, they rose in 1832 before that fizzled out, and what do you expect the Amis to do with a popular revolt sweeping all of Paris? Stay home? Join the ranks of the republicans who abandoned the people because they figured the odds weren't certain enough, and in so doing doomed the June Rebellion? They're not exactly setting up camp with a single barricade in the Rue de la Chanvrerie, hoping the rest of Paris will get the idea. Paris is in rebellion, and it's their duty to fight alongside their fellows.
As for that "the twentieth century shall be happy" bit... Enjolras here is no more naïve than Hugo himself, and Hugo's predictions for the twentieth century were more nuanced than fuzzy-bunny utopianism. He predicted, for example, the end of war as a formally-declared clash of armies, and predicted that that model would fail because both sides would possess weaponry of such mind-blowing destructive power that neither would dare use it. When he said there would be no more history, he meant history as nineteenth-century Europe understood it: formally-declared clashes of armies, power struggles between dynasties carving up the map to suit their feuds and alliances. I think he thought the 20th century would largely manage to fix the problems of the 19th and go on to develop more advanced problems of its own that the 19th century could scarcely conceive of. It's actually a pretty perceptive prediction, especially in some of the details (mutually assured destruction as an end to war between great powers!)... at least where Europe is concerned. Outside of Europe he was awfully fuzzy on the details and just theorized madly, which meant his opinions on (say) the future of Africa were both horrifying and completely out of touch with reality.
[Dieu] entend ta voix, ô fille des hommes! aussi bien que celle des constellations; car rien n'est petit pour celui devant lequel rien n'est grand.
- George Sand, Les sept cordes de la lyre