They can and that`s exactly what TLJ did - every set up,every established rule,characters,relationships - changed,led to nothing,cut off short - Technically The SW universe/franchise did not fall apart,it continued but it became something else entirely.It would be like starting different game in a middle of this one.
No my problem is not realism nor my subjective taste,what I like or dislike,want or don`t want to happen,its being consistent with what you established,following through.If you establish character as smart and suddenly that character starts acting dumb than thats bad writting,now if the same character would do something horrible but consistent with that character - I might dislike it but it wouldn`t breake the story or made the writting bad.