The Original Star Trek series wasn't quite socialist tripe. If you notice, even the space hippied episode had the space hippies die at the end from their foolish and impatient idealism. It had episodes that were pro-Vietnam intervention, pro-life, anti-collectivism, and so on. It's anti-racism episode showed the racism as a two-sided problem and at least two episodes had themes that children require adult supervision, which is quite far from the clever kid themes so common now. Basically, the still-conservative values of the mid-1960s moderated the show quite a bit and it was actually pretty conservative.
Compare this to a show like Babylon 5 which had less "great" episodes, but which was consistantly "good". (I can't think of a B5 ep which was a true stinker).
If you really want to see some socialist tripe, see the first season Babylon 5 episode, "By Any Means Necessary", a love letter to organized labor. Yes, the "Rush Act" is meant to mock Rush Limbaugh. To top it off, it's badly written with a straw-man villain and laughingly heroic labor union workers. There were some other awful first season episodes, like "TKO". And let's be honest, the fifth season wasn't necessary. I do think Babylon 5 was consistently good, but it wasn't stinker free.
I was always hoping they would do something like the Wrath of Khan again, but I don't even think Rodenberry liked the Wrath of Khan because it was too "militaristic". Horatio Hornblower in space was really what Trek was meant to be, not bald French UN diplomats in starships or insufferable, preachy know-it-alls like Janeway.
Yeah. They didn't know if there was going to be a 5th season, so they had to end season 4 in such a way that it could be the end of the series. Season 5 tried to spin things up again and it didn't really work. Still, "Severed Dreams" is probably the best sci-fi episode I've ever seen. I get all tingly when Mira arrives with the cavalry.