SMBC today is particularly appropriate to AW. If this had happened would your game be the dream or what was left over after the vast majority of people gave up operating the real world?
For me the obvious thing to do would be to lay out the story about how the world ended and let the PCs argue about whether they are dreaming or real.
If you’re dreaming everyone else is just a figment of your imagination of course. Or maybe you’re a figment of theirs. If you are dreaming do you want to wake up? If you are a figment then the bad things that happen to you are really caused by the dreamer,. What are you going to do to them?
Great example of “take an interesting idea to the logical extreme”.