This episode has hallucinogenic qualities, so I wouldn't try to interpret it in terms of strict scientific possibilities. I always found this one of the creepiest of Twilight Zones, esp. in a philosophical way, i.e. "Do I exist & how can it be proven objectively". The story maybe advanced by the idea that the astronauts passed thru some kind of mysterious & destructive time warp.

Still, the concept of disappearing time/existence might work easier without resort to the whole spaceship/astronaut conceit, that seems to demand more explanation than necessary. Imagine simply that there are holes tearing up the time/space fabric, or a fold in time that keeps folding over again & again, swallowing up existence of people & things, like some kind of black hole that keeps sucking up our existence & understanding. Far out -- not an episode to watch on drugs.