EDIT: Oh, right, the trolls DON'T know what went wrong. Maybe they are wrong and the Rift happened independently of all this. Maybe the stupid game is unwinnable. Congratulations, Skaia saved, a winner is you. The timeline will collapse in 4:12:59:57. The purpose of the game is still a mystery. Save Skaia, the source of all creativity. Then what? What's supposed to happen when you do that? What happens if you fail?
I'm fairly certain that Skaia is one of Lord English's engines. Winning requires several stable time loops that we've seen; for example, we know that Karkat is responsible for his friends' existence in the same way John is. Speaking of which, I realized who the wild card is:
Mr. Egbert.Grandpa and Mom both helped with the development of Sgrub. Either they are not fully aware of its purpose, or they were deceived as to what it was for. As well, they were both born due to ectobiology, as is every single one of the major pieces in Sgrub, all except for Dad. Is Dad the wrench in the machine? He's the only one who is so far seen to be unaffected directly by the weird time shit, and he has been investigating the meteors for a while. Could he be the breaker?
I'm still going with my "Jack is the game breaker", of course, because that's the one that makes the most sense to me. This is due to the conjunction of the First Guardians with the apparent destructive nature of Sgrub/Sburb. A Guardian's purpose is to shepherd a planet towards intelligent life, right? Which would entail developing or discovering the game - reinforcing the stable time loop that ensures that Lord English
will be summoned at the proper time.
This is probably what his Cairo Overcoat is for, to monitor all the tears in paradox space and ensure they are fixed.
The destruction of Alternia and Earth isn't something that happens if they fail right? So it is basically inevitable, like Nannaquin says. So I don't know where that fits in. But it's been stated that the point of the game isn't to save the world, unlike what Sollux and John thought... Yeah, I'm thinking this is partly Lord English's doing.