Well, actually, the entire idea of SW through the OT and the PT is the story of the Skywalker family.
So, if you're doing a sequel trilogy to those six movies you really have to give closure to that family's story... If you don't want to do that, or if you think that it's not necessary, well, you're wrong.
The ST started off as precisely that: the chance of closing the Skywalker chapter and setting up a new group of characters. TFA did that fine: it gave Leia and Solo closure and set up Poe Dameron as the cocky, self-assured "scoundrel", set up Rey as the new heroine (the role previously occupied by Leia) and set up Luke for the "mentor" role. But then TLJ screwed the entire thing up, because by killing Luke it took away the chance of closing the Skywalker Saga satisfactorily. Sure, he may come back as a ghost, but he never did pass on the torch. He actually refused to do so.
What happened is something like this: you're telling the story of the Corleone family. You start off with Vito, then Vito dies and you continue the story with Michael and then, instead of having Michael teach his nephew the ways of the family, you kill off Michael and pick up with one of the grandchildren of the Tattaglia family. It makes no sense. It goes against the entire idea of why these movies exist. You don't make a movie about Tattaglia or Barzini and call it the Godfather 3.
If you want to do that kind of thing, you set up a story in the same universe and call it "A Star Wars Story". Like RO.