Length of the source material or it even being some kind of trailblazer back in the day isn't really my concern here.
Spoiler: showPersonally, if we found out there were demons, aliens, or even freakin' gnomes in our dryer that steal our socks, I highly doubt the world would go to shit that swiftly and easily. The concepts of fears and irrationality were pretty much jacked up to 11 with conveniently hilarious plot points like Russians making the demons so let's go nuke 'em. Oh, and fleet leaders have been demonized, but no one noticed them looking/acting differently.
Ryo being Satan (with tits?!) might've rationalized some of his behavior up until that point, but it still feels like a pretty convenient asspull to push things into the ending. Just gets topped off with the "God's a dick!" trope by nuking the Earth.Spoiler: showWe can barely handle different races of people (hell, even the same lol) and you think we could handle demons or aliens with any level of composure?
Otherwise, the rest was easily explained and actually true. People listen to authority figures without much proof/evidence, and fear and suspicion brings out the worst in people. The rest is literally just "demons did it" mixed in with the rest. With demons actually showing up and shit and I wouldn't doubt the world would go to hell very quickly.
Ryo being Satan is the other reason for this happening. He even says that he was unconsciously heading towards this end. Hell at the start he's the one acting weird but they make you forget that he started this whole thing by being crazy. And him being a sociopath is one of the first things they show you. The only thing his relationship with Akira brought him was a sense of love and then a sense of loss and pain. It just feels like you weren't really paying attention when your gripes are things they explain and foreshadow. I'm kinda glad I watched it on netflix because holy shit does the OP bang that concept into your head for a while.
Devilman is probably responsible for that trope even being a thing in Japan too.