I don't think anything approaching serialization in nightly programming existed, until '70s-era mini-series like Rich Man, Poor Man
started showing up. But those were usually limited to telling their stories over about a month's worth of airdates, and were promoted as big events.
I was never a big fan of serialized shows, because you had to watch them every week in order to keep up. You could miss one episode, and be completely in the dark about what's happening over the next few weeks. This is the problem I had with DS9 and ENT. I've not seen any of the shows that became popular in recent years, like Sopranos, Mad Men, Deadwood
, or Game of Thrones
, as I can't pick up regular television since digital broadcasting became the standard. The only way I would ever see these shows is if I decide to pick up the series collections.