So some god smoked a million godly cigarettes and exhaled that shit all over the Earth, causing the entire planet to erupt into chaos, death, explosions, implosions, and general apocalyptic events. The air is foul. Probably poisonous. Despite this, life inexplicably continues for one slacker and his cat. As far as Isaac knows, and depending on what side of the AI sentience debate you're on, he's the only human left on the planet. He would resign to his fate-playing the latest Fallout or replaying Portal 2 on the PS6 until the electricity fails and he begins to rot more than just his brain-if not for the four legged creature demanding canned tuna and a clean litter box.
In Ashtray, hell isn't other people, it's the inanities of technological convenience people created to fill the void. It's the realization that even if you manage to outlive the end of the world, you will still be subjected to hold music.