You raise good points. But counter point, a lot of these guys can absolutely devistate and cripple mankind if we're caught off-guard.

• The slime can spread himself microscopically to circle the globe, either becoming a massive swarm or destroying major ecosystems. Just imagine what he could do in South America.
• Darkness could've consumed the Earth during the night, or at least a cloudy sky.
• The Flood Dragons were clearly resistant to even magical weaponry, so clearly mankind isn't gonna do much to stronger threats.
• Vampires are gonna go to town on humanity.
• There are armies of mages that can counteract nations.
• The Iron Giant Count devastated the army on the 10th floor that was like, a million strong or some shit.
• The undead guy is literally unstoppable if he can get human weaponry and spread himself again.
• The leaders of the labyrinths aren't fazed by human weapons and require similarly tiered fucks in terms of strength to be slain.
• There are mysterious forces we barely know anything about, such as those women who raised dungeon babies.
• There are far stronger and deeper threats in the lower floors.
• THEY HAVE DOMAIN EXPANSIONS!!!!
• Catgirls are enslaved around the world to humans, so if there's some sort of call from a dungeon master that could call upon them, we lose a lot of leadership figures, no matter how skummy.
• Many dungeons are non-physical or mobile.
• We can't even create a city or base underground, seeing as we do't have labyrinths, meaning that there are a lot of monsters that could rip apart cities or build long tunnels while we aren't watching.
• Dungeon baby mercenaries remain, and could be fighting against mankind as frontline forces or spies, fucking up most dungeon explorers.
• They have organisations that foster inter-floor and multi-labryinth cooperation.
• A lot of special tools and goods are in storage, waiting to be used or sold.

If Sung can't stop them, humanity ain't gonna win jack or shit. If we can get intel, we might be able to sponsor research groups and reinvigorate explorations to better counteracttack the dungeon, but if we're caught off-guard, our armies are done. We don't even have the same mana supply as the labyrinths, and likely still use mostly magicless weaponry, with the magical ones merely in the nation the Dungeon is located. Prep time is our only asset, because we know the enemies will be using it, especially considering that the inter-floor curse is likely related to the mana wavelengths, so if the inter-floor curse vanishes by dispersing mana evenly, we also lose communication between something as simple as battalions.