I've been using Freevee to watch Magnum P.I. on our Roku. And there are about 10 broadcast rerun channels if you go over the air here.There may yet be hope for those old shows that you could watch on some obscure UHF channel, back in the '70s and '80s. The free streaming services, like Tubi and Roku, need cheap content. As a result many of those old shows are being revived and put online. For example Tubi has the old Batman and Captain Marvel serials, The Addams Family, and many others.
The big difference is that back in the late 70s early 80s a lot of places only had one rerun channel and there wasn't cable -- so they had a lot more traction.
This has me thinking about Saturday Morning Cartoons. I wonder where Tom and Jerry and Looney Tunes would be in the cultural memory without the decade+ of their reruns in the Saturday morning loop with no real competition. (Back before it was taken over by mostly new shows).