FOTM trash like Demon Slayer comes and goes, and when the anime is done milking and the soulless final season drops, it will fade away into obscurity as another one of those forgotten bestsellers. It will never stand next to actual eternal legends like DBZ. Some alien archaeologist can cracksl open an old hard drive, boot up the first episode of DBZ, and before you know it, their entire species is obsessed with Goku. They'd be screaming "KAMEHAMEHA" on their home planets, debating whether Ultra Instinct can beat their warlord's most powerful general, and designing their own scouters to measure power levels.
Meanwhile, no alien is sitting around watching Demon Slayer and crying over the moralfag MC's dead family or listening to some demon's sob story for the billionth time. They're not going to sit through Detective Conan aka Japanese Scooby Doo or Slam Dunk when they could just as easily watch the dominant 90s Chicago Bulls era instead of having to read about some Japanese high school brats loosely based on them.
They'd be watching DBZ because that shit is primal. It's about limitless power, about pushing beyond limits, about screaming until you break the laws of physics. That's the kind of universally appealing insanity that transcends time, language, and even species. If anything survives the collapse of human civilization, it's going to be:
DBZ
Football
Pro wrestling
Pokemon
Mario
Michael Jackson
Jesus
And maybe a couple of fast food mascots because capitalism never dies