Using Nocturne as an example was a terrible idea.
You wrote a cool story, bro. Guess what though?
That's a different entity entirely. It ain't Nocturne as he existed. Previously, he was the embodiment of a plague that affected us, the players, something we could actually roleplay as if we feared it even as we controlled it. We could be scared of Nocturne and feel throughout the game as if we were playing with fire (and I don't mean the Brand sort of fire).
What you've described. Just some spooky ghost that kills people on some fantasy land. It doesn't scare me. It's some freaky abomination, sure, but what's it to me?
You are making the same mistake Riot is in thinking that just because you can describe something in a cool way with eloquent language, it must therefore by default be intriguing and compelling. Maybe you didn't roleplay yourself as a summoner, but some of us did, and that is why any new Nocturne that works in this new lore can never be as compelling as the old one.
About why Nocturne was a terrible example for your point: there are plenty of these characters that CAN be written into the new world to be as compelling as they were before, but Nocturne is not among that number for the above reasons. The main problem with Riot's decision though, is the simple fact that - as is the case with Nocturne - many of these characters' identities will in some way need to be altered, and in many cases this may change them away from what might have drawn the current players of that character to that particular one. There's a lot of hatred for past lore rewrites, and that's why. How do you think rewriting this many AT ONCE is going to blow over? It's a mistake of colossal proportions, and this is how they "announce" to us they're only "just getting started." It's highly discomfiting to put things lightly.