Of course, the DM could also decide that when everyone in the party reaches 0 hit points, they are routed instead, and immediately switch to a chase scene. Or maybe everything fades to black, and then the party wakes up to realize....
...they've been robbed and left for dead?
...they were captured and dragged back to the enemy lair?
...they've been rescued by a good Samaritan, who cautions them to be more careful?
...they've been rescued by a devil, who is expecting payment. Right now.
...they've been rescued by a necromancer who...aw man, this isn't a rescue, is it?
...it was all a dream?
...they're ghosts now?
...that sword they found in the last dungeon had a special, life-saving power?
Death is a fine option, sure. It's certainly the most common. But it's hardly the
only option, and that's been true for every edition of D&D.