Superheroes aren't supposed to die. They're not supposed to lose their powers and get old like the rest of us. They're supposed to win and keep winning.
And they did win. They stayed young and strong. The bad guys lost, and we waited for the next issue. This narrative was working fine until M. Day. Somehow, the bad guys win, the superheroes lose, and we're left waiting for Wolverine to wake up and say it was all a dream. But there is no dream. This is the Marvel Universe's Earth-616.