Dallas had no planned arc and no real end in sight. Soap Operas just keep going until the money dries up. The same went for Hill Street Blues, though it was far more sophisticated than soap as I recall.
As for Flash Gordon etc, they are shorts that, as a story, congeal together to make a roughly hour long self-contained tale.
A lot of it, of course, is that B5 (and all the shows like it) could only exist in an era of video and dvd sales. In fact, shows have become more complex and arc-driven in response to that technology.
The only show of the above mentioned I watched regularly was Hill Street Blues. My sense is that the story arc would have been season long, but that they were quite okay to refer, tangentially perhaps, about other things that had happened in other seasons.
I had noticed the increased complexity in television series but it didn't dawn on me that it could be related to dvds and other retrieval mechanisms.