Well the Dungeons only appeared since 10 years, and since there is South Korea, I think it's implied that the world evolved in the same manner as ours. The existence of these "Gates" and the fact that they consider the creatures inside as monsters and mythical creatures, hints that their knowledge should be the same as ours if ou world suddenly spawned Gates. So I can understand that the middle-aged men wouldn't be prepared against such things, since they probably didn't play games like tabletops RPGs and such. But the young ones? A gigantic room, different statues everywhere, lamps with blue flames that lit spontaneously, RUNIC TEXTS?
How the hell could they not have seen that coming! xD
And... really? Dismissing other people's opinions and remarks as "negative thinking"? My favorite is their lovecraftian-like excessive rationality and refusal to consider magic and possible supernatural things, even when they throw fireballs xD