Funnily enough, Marvel actually use both explanations. Yes they have the infinite multiverse where every possibility has happened and forked a new timeline, however they also have a sliding timeline in the prime universe where big canon events are always shifting to keep the existence of superheros within the last twenty years or so. This is why characters like The Punisher, War Machine, Reed Richards and Ben Grimm, whin were tied to Vietnam in their origin are now veterans of a fictional war that can be moved up the timeline. DC actually uses a the multiple-multiverse concept where each one only contains 52 universes, and travel between them was limited until recently.