Buffy was not focused on relationships and teen drama. It had those things sometimes, but it wasn't the purpose of the show. In the end, Buffy was about the slayer and the things she did, like saving the world. Sure there could be drama, and there were definitely relationships (which ranged from terrible, like Buffy and Riley, to well done, like Willow and Tara), but that wasn't the focus of the show. Stuff like Teen Wolf/The Originals might have overarching plot points (I'm just guessing that they do), but the focus is on the relationships and the drama, they're the core of those shows and the reason people watch. No one tuned into Buffy to see what was happening with her love life, they tuned in because the show was about good characters getting into trouble and saving the world. It was well written (generally, all shows have bad episodes), and while characters were a big focus it still did good stories and arcs.
It all comes down to this: romance/relationships were an element of Buffy, but not the focus (and they were generally done about 1000x better then the angsty romances of modern stuff). Stuff like TW, TO, etc are all about the drama and relationships, with some supernatural fluff added for variety or to set up specific types of angst/drama. Also, the idea that The Originals is more adult then Buffy is laughable, but then again I find most "adult" labeled stuff to be fairly childish and immature, so maybe it fits. I'm not even going to acknowledge the comparison to Teen Wolf, which is about as ridiculous as comparing Buffy to Twilight just because they both have vampires.
Also, since I was talking about stuff I like, good Sci Fi/fantasy is mostly not there. It just doesn't exist, outside of superhero shows and very rare special cases like The Librarians. The Sci Fi/Fantasy world is ruled by angst and drama. I just want good characters, good writing, and interesting stories. Those are three things that aren't done, because a very specific demographic just wants shows about angsty brooding people and their relationship drama. I hope that changes, but until then its a terrible time to be a fan of fantasy when it comes to TV shows, and outside of superhero stuff Sci Fi is just about as dead on TV. If people enjoy angsty drama, I'm happy for you. Same with people that like the violent semi-porn of HBO or stuff like The Expanse. But I hate that stuff, and obviously the stuff I like isn't being made much. I hope we eventually have a new golden age, where stuff like Teen Wolf and The Originals goes away and we get stuff that does Sci Fi and Fantasy as well as they used to. Things more like TNG then Expanse, or more Buffy then Vampire Diaries. We probably will at some point, the angst/drama has to burn itself out eventually, and I won't believe that real space operas are totally dead.
But, as of right now, TV is pathetic when it comes to non superhero Sci Fi and Fantasy TV (well, its really just pathetic in general, but Sci Fi and Fantasy are the stuff I mostly care about). To me, its all unwatchable angst, drama and grim darkness in Fantasy, and Sci Fi just mostly doesn't exist. I can't wait for that not to be the case, but I think it will be awhile.