Do we, in our hearts, no longer believe in Utopia? Indeed, did we ever pledge ourselves - genuinely - to Gene Roddenberry's teachings?
Gene Roddenberry's "teachings"?

Dude. Much respect for the Great Bird of the Galaxy and all, but that's a bit overboard. Roddenberry didn't have any "teachings" or any cohesive utopian vision. "Gene's vision" was about making good television, and Star Trek's "optimism" was in the beginning just a matter of dramatic license -- Earth was to be kept off-screen and portrayed sympathetically so that we could cheer for the heroes, that was pretty much it. It was a good idea but it wasn't a transcendental calling.
TNG's "utopianism" and references to "paradise" (which as you can see a lot of people found obnoxious) was just an exaggeration of what went before, and while I personally do feel that it had a certain charm, overall it was not the best of dramatic ideas and it's not something I miss too terribly much.
(Also, the word you're looking for is "disenchanted.")