TOS and TNG were basically episodic and that format makes it easier for new viewers to jump in midstream. I think that's partly why those shows can still work well in syndicated reruns because you don't have to watch them from the beginning.
TV is different today. People don't watch syndicated reruns unless they're just looking to have something on. The real way that people get content that they want is through Netflix, Hulu, or DVR type stuff. And those methods allow people to binge watch more than ever, which is why most of the latest greatest shows work as being arc-based. That doesn't make episodic shows out of date or anything, but a show being arc based does not indicate anything about if people will watch.