Every series is going to have a few episodes that work better than others going into the future. Some TNG much less TOS seems laughable now. Hell, even some ENT felt dated from the time it aired. I think that TOS is quickly reaching a retro equilibrium where the episodes that feel dated now and will feel dated in, say, 10 or 20 years will be virtually identical. TNG is in its phase right now where it feels more and more dated, especially the first two seasons but not excluding the whole. Since TNG is starting to go through it now later seasons are more immune to the effects, not just for being made later, but also becuase the writing and production moved in a different direction. DS9 through ENT are largely too recent to feel that dated. Early DS9 and VOY have a few bits that feel very 90s, and ENT has some things that feel very married to 9/11. Still, yet, I think that in 20 years this will be how things will be:
TOS: Won't feel terribly different from today. Still kind of a retro chic vibe, not a serious image of the future today or in twenty years.
TNG: I don't think TNG will ever be exactly where TOS is today. Many things will look more and more silly. TNG had a more timeless aesthetic in a few respects, but also really, really didn't in others. It still will be a more consistent universe than TOS since its writing staff made more of an effort to develp a mythology. Seasons 1 and 2 willl feel even more dated than they already do, which is quite bad. Still, like TOS, a lot of it will be episodic. I guess what I'm trying to say is fewer of the episodes will be so tied to their time than TOS, but still many will be. The general production design is a little more timeless, but will still show its age.
DS9/VOY: One of the few times I can ever lump these two together. They didn't exist during early TNG so they never went through that phase. Their production design might hold up decently for quite awhile. I see both becoming more irrelevant for different reasons. TNG had a lot of what VOY had but with more pop cultural appeal. DS9's mor serialized formula is being copied and done better by other shows. I think it will be awhile before either show feels completely retro, but they may not be so important and DS( will feel less fresh.
