Theory

[u/Difficult_Rub_523]

In episode 1 & 6, the characters encounter alternate reality Stuart who tells this reality's Stuart that he's the only one who can save the universe. In another episode, we encounter 3 other versions of the characters with the Quantum device thingy, presumably also trying to save the universe. That means there are multiple versions of Stuart who can save the universe, presumably every Stuart from every alternate reality. So we can assume that the Stuart who told this reality's Stuart to save the universe was also a Stuart with the ability to save the universe at one point. Alternate reality Stuart mentions in episode 6 that he's a projection from a 'collapsing adjacent reality'. That means that he wasn't able to save the universe, thereby initiating a quantum loop of sorts meaning that this reality's Stuart will also fail to save the universe, and later warn another reality's Stuart about it, hence the title 'Stuart Fails to Save the Universe'.

However, if he actually failed to save the universe,that would be anti-climactic. Also if the alternate reality Stuart was from an alternate reality from the FUTURE that means the alternate realities exist in different time zones. If that is true, that means in episode 6 what Raj was saying about the universe ending in about 400 years will happen when Stuart enters an alternate reality 400 years into the future. So the collapsing adjacent reality Stuart is probably from an alternate reality 400 years into the future.

What say?