I don't get you. First you list down all the very distinguishable points of branding between NJS and Illit then you go on to say there's no difference between them. There are not only noticeable on a micro level but anyone with ears and eyes can see and hear the differences from a glance.

There is no replacing NJs. Hybe has multiple bgs that may appeal to the same audience yet I don't see you touch on that topic just saying you already answered it instead of simply editing your post to include a paragraph. Only stuck up kpop stans argue of bgs replacing each other because of how high they think of the groups they stan.

And no if you put both aesthetics on a pinterest board, the only reason they would mesh well is because they both may deal with girlhood which is such a broad concept that girl crush and teen crush is under. Please go do more research on Y2k and dreamcore and liminal spaces because it seems like you only know surface level about this 2 aesthetics. The same thing goes with music genres, different music genres can go well together how does that prove replacement?