So you believe in some god-given order of genres that is ever unchanging. Because turn-based RPGs were popular at the time and the first developers decided to call their new gameplay in real time also RPG, you think it is all the same. Whatever.
Whichever genre a game is in or not in, you must accept that different gamers like different games. As real time and turn based is fundamentally different gameplay, it is obvious that some gamers prefer the one gameplay, others the other. The problem is you demand that fans of one gameplay have to support games of the other gameplay, because you put them for whatever reasons in the same genre. And I don't see it. Support games you like.
As turn-based fell under the radar a bit over the past years I can understand someone celebrating the success of Octopath Traveller, as it is proof that turn based games have their fanbase and can sell good, if the game in question has quality. Which is what people don't understand, they say some cheap turn-based games sell bad and conclude the genre is dead.
And to your last point: no, I deny that FFXV paid for Octopath. FF was so incredibly expensive, that it was a high risk. It did pay off, but it could've been different. But that all doesn't affect Octopath much. The development of Octopath was comparatively cheap, so lesser profit would've been enough. Moreso, the genre is completely irrelevant if you ask what pays the bills. So I could say Tomb Raider and Just Cause paid for Octopath, because Square don't care. Most of all though, Bravely Default already proofed that turn based RPGs can sell. It was made by the same studio. So - if at all - it was Bravely Default which made Octopath Traveller possible. If Bravely Default had failed, Octopath wouldn't be here. And with these sales Octopath paid for itself.
So in the end I don't need to support a shooter, an aciton-adventure or whatever to get my turn-based RPGs. I support turn-based RPGs to get turn-based RPGs. That's why I bought Etrian Odyssey, Bravely Default, Shin Megami Tensei, Persona Q and Tokyo Mirage Sessions - because I wanted to play these games. And with that I signalled I want more games like these. Some different games don't influence that.