Lol... I think in all fairness, Marvel may have been the gateway to get the rest of us into it.

They've had a better grasp on sci-fi and fantasy longer than the general public, and since I'm not a comic fan I can't tell you which edition of which series it appeared in first, but I bet the concept of something that resembles a metaverse could be found in print from the 60s or 70s... maybe earlier, but one may have to bounce over to DC to get that far back...