ST:TNG and B5 were both first-run syndication, and are more-or-less irrelevant to the argument. Dallas was one of the most popular TV shows of all time. It's not surprising that they tried to milk it. (In comparison, NBC ordered 30 episodes of The Office last season, but it got shortened due to the writers' strike.)

In general, though, shows used to tend to run 24-26 episodes per full season, and now tend to run 22-24 episodes per season. Of course, you don't recall all the failed shows of the past, which also didn't run full seasons. Just randomly picking 1979, Delta House (15 episodes), Supertrain (9 episodes), Out of the Blue (8 or 9 episodes), Working Stiffs (9 episodes, only 4 shown).

And I agree about the split seasons thing. It's obviously so that the cable channels can run their new episodes in times when network shows are in reruns, but they did it as awkwardly as possible.