Humanity, at least the front face of it, is nowhere near advanced in comparison to Honkai Humanity. So unless the whole Mage's Association and a good, sizable portion of Atlas come out to do some cool shit, we already start off in very bad footing. Besides that, biggest issue I'd say is whether...
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He really is underestimating alot of these goddamned groups, isn't he?
Atlas alone has the Seven Taboos which is a danger to the World(Which would still be Planetary+ regardless, thanks Textures) itself:
This is ignoring the Burial Agency if we are including Ciel and Calvarro Star, Kiara Kisshioun if she's allowed(dear fuck) or the MA as a whole including the True Wandering Sea or if you want to include the Dead Apostle Ancestors or the Tsukihime/KnK cast minus Arcueid.
Why would Humanity need to harness the Honkai? That makes no sense with how either series tends to work. He's again giving little doubt to the Mages, The Church or otherwise who would absolutely find out and want to deal with the Honkai in various fashions.
Why is this dude going on about with this fan fiction nonsense?
Yeah, then the Planet starts really cooking and Honkai ends up in a war it cannot conceivably win, even with Herrscher assistance...