The fact is, IMHO, there is
almost no plotline that you can't do -- short of flying saucers -- depending on how it's executed. But when a series is just slinging sloppy ideas to the audience, sloppily rendered, even the most potentially-inspired story or new set of characters just can't work.
The reason DYNASTY got so bad, after Season 2 or 3, weren't the story ideas but their implementation where the dialogue flow became goofy and the actors became overly-constrained causing them all to seem victims of a group-rate lobotomy. FALCON CREST was always a show that could get away with being crazier (and that's where the UFO should have happened) but even that show started to feel too stupid when plots, however expectedly wacky, became too wink-at-the-camera facetious (like what happened to Melissa, once perhaps FC's most popular character, was made increasingly mentally-ill for the purposes of "laughs").
At least KNOTS played it straight until the end, which is why its ratings outstripped DALLAS and DYNASTY by the end of the '80s, even though budget limitations saw the core KNOTS characters replaced by a daytime soapy barrage of younger people in its closing years.
Cally, James, Michelle, Vanessa -- even ELLIE, SHE WROTE and Lady Jessica's unladylike murder-spree -- could have worked had the final seasons of DALLAS remained character-based and not shifted into the whimsical,
'oh-what-the-hell' tone that it did.
At the end of the day, these programs were essentially family dramas, and when they got away from that, they mostly all fell apart.