There is a lot of overlap.
Now this, of course, doesn't mean that BTS ain't the biggest bg yet but there clearly is fandom movement. Which shouldn't really surprise anyone, it always happened. Happened to EXO after NCT debut. Happened to Twice after PD48 and Itzy. Will happen to BP once Babymon debuts, and if it is good.
But I just find it odd that it was SK not TXT or EnhywhateverIalwaysforgethow tospellit, Bighit fucked up big time by trying to make groups different and not debuting a true successor to BTS.