From making something that anybody can jump into at any time and enjoy/understand, to creating a world so intertwined that if you miss anything, your understanding and experience is markedly poor.
I've just checked and the episode was produced in the third season. And according to the various synopsis I've found, This aunt was very present in her life and they often went out together for shopping, cinema etc. We just have to accept that they met between one episode and another and by a series of coincidences Mallory had never mentioned her in the episodes broadcast (except when she died).
"Last century" is a huge overstatement. Shows often had a good amount of continuity by the '80s and '90s. I'm finding that The Greatest American Hero is quite good at continuity with things like character arcs and referencing prior episodes, even though it makes changes to things like character names and has inconsistent chronology. There is a weekly reset button where the plots are concerned, never really changing the episodic status quo even when the heroes get involved in events that you'd think would have massive consequences, but in terms of character details and arcs, recurring guest stars, running gags, and the like, the continuity's pretty solid. Plus, of course, it was the '90s that serialized plotting began to emerge. The only thing that's really different in this century is how much single overarching season-long arcs have come to dominate. People are far too prone to equate continuity with serialization, but they're two very different things. Serialization is telling a single long story; continuity is just having individual stories be remembered and have an impact on later stories. 21st-century TV is not better at the latter than 1980s-90s TV (or even 1970s TV to an extent), just more addicted to the former. Also, saying TV is "better at" it is imposing a value judgment. TV back then was made by people who were just as smart and capable at telling stories as people today, but the nature of the medium was different. In the '50s-'70s, there was no home video, no wikis, and often unreliable broadcast signals. There was no way to guarantee you'd see every episode of a show, so it made more sense to prioritize making each episode complete in itself than to prioritize making them chapters in a larger whole. There were shows with serialization -- soap operas, children's adventure shows -- but they were basic and repetitive enough that it didn't matter so much if you missed some chapters. Back then, episodic TV was seen as smarter and serialization less sophisticated. It's only become inverted now because it's become easier to experience a series as a unified whole.
What was the ratio of anthology/travelling hero shows (different characters/setting/time period every week) to "cast of characters/stationary hero character(s)" shows back then?
Well, there was overlap between the two, since even shows with continuing characters aspired to an anthology-like approach. Star Trek is a classic example -- a fair-sized cast in a permanent location, but the location was a ship that went to a different planet every week or dealt with different visitors or guest crewmembers every week. Ditto for any detective show, courtroom drama, or medical drama with a significant-sized cast dealing with anthology-style cases of the week brought to them by guest stars. Then you've got something like Mission: Impossible, where there's a fair-sized cast, but they not only travel to a different place every week, they adopt different personas every week, making it even more anthology-like. Even when you had a sizeable cast, after all, the goal (except in soap operas) was to make each episode a complete experience that the audience could enjoy independently of other episodes. So there wasn't going to be a great deal of change in the characters' lives, and the focus would still be on self-contained stories. Of course, the advantage of a permanent location is that you could use standing sets, while anthology shows like The Fugitive and The Incredible Hulk might need a lot of location work. (TIH was very expensive because of its lack of standing sets as well as its ample stunt work. At one point, the network tried to convince Kenneth Johnson to give David Banner a Rick Jones-esque sidekick with an RV they could use as a permanent set.) On the other hand, a lot of anthology and anthology-esque shows did use standing sets, just redressing them from week to week. For instance, M:I had a corridor set that often showed up as a prison or hospital interior, and The Man from UNCLE had a mansion set that showed up frequently as various different mansions.
Yeah. And several episodes of Perry Mason used the same staircase over and over. (A hall going across to the right as you face it and down one flight of stairs on the right side next to the wall.)
Apparently, we have I Love Lucy to thank for this episode, too; their Christmas special is (supposedly) the first ever TV clip show (movies had done this previously). https://ilovelucyandricky.fandom.com/wiki/The_"I_Love_Lucy"_Christmas_Show https://en.wikipedia.org/wiki/Clip_show
Not just movies, but also movie serials. It was common in serials to save money by doing a chapter in the second half that involved the characters recapping the events of previous chapters, either to review the case so far to try to put the pieces together, to explain the situation to a character newly brought into the loop, or the like. So the clip-show principle probably originated there. Serials would also recycle footage in other ways, like replaying the previous chapter's cliffhanger at the beginning, sometimes several minutes' worth. Or they'd be written around stock action footage from earlier serials or movies. And of course they'd use stock scenes of things like the hero going into his lair, donning his costume, and riding out on his horse or in his car -- basically the forerunner of how superhero shows include stock transformation sequences or giant-robot assembly sequences or the like.
You know of no other clip show television episodes aired prior to I Love Lucy's, then? (After all, the Star Trek "first interracial kiss" claim is ambiguous at best - let's not get into that topic, only mentioning it as an example of where fans have been wrong before)
I didn't even know Lucy's was claimed to be the first. Though it's a plausible claim, since the show did pioneer a number of things.
Neither did I. I don't know it it's an official claim or original research/guessing on the part of the person who said so.