plane shifting vs different settings

[Castamir]

So... we know that only a single planet on the Material Plane holds the Snarl, yet there's more than a thousand planes (http://www.giantitp.com/comics/oots0633.html) that mortals can reach. Thus, people can go back and forth between whatever planet (and setting) happens to currently exist. Another way to survive is plane shifting — Astral is a safe place, most other planes probably too. And a world doesn't go away in an instant (http://www.giantitp.com/comics/oots0274.html), giving any mid-level cleric, high-level wiz/sorc, and some outsiders — plus their party mates — plenty of time to escape.

The exact kind of the catastrophe or an explanation for it might be very hard to discern (there's nothing to compare the escaping witnesses' description to), but at least the existence and destruction of past worlds shouldn't be a completely unknown information. Together with some people/talking animals/animated snacks who managed to survive and thus are living proof.

So why didn't any of visitors who shifted back share this knowledge?

[KorvinStarmast]

So why didn't any of visitors who shifted back share this knowledge? 1. They didn't care
2. Nobody listened. (See Greek Mythology, Troy, and Cassandra the prophet / seer).

[Emanick]

Alternatively, maybe people did listen, but it’s been long enough that the few people who heard about the old world have long since passed on, and the stories about the previous world live on only as obscure myths that few people have heard.

[Sorator]

That assumes folks know to escape in the first place, doesn't it? It's not like word will spread around the globe in the <30min it takes to undo all of creation. And the number of folks theoretically capable of casting a specific 5th or 7th level spell given time to prepare is not at all the same as the number actually prepared to cast it with almost no notice whatsoever. Chances are there's very little time between someone realizing there's a completely unexpected threat and that someone being destroyed by the Snarl; we've seen how quickly it kills folks through the rifts. Any potential caster would need to have Plane Shift prepared & have the necessary tuning fork component for a safe plane, and process the danger (not a small obstacle, given the lack of general knowledge among mortals re: the Snarl), and choose to use said spell instead of trying something else (how many folks would try to fight the Snarl instead, or move elsewhere on the same plane, or otherwise just do something else?). And that's assuming the Snarl wouldn't somehow prevent such an action outright. That's a lot of things that have to go right.

And then they'd have to know that there was a new world to go to afterward, which is also not a given. Not to mention that the gods might prevent any escapees from returning, or from escaping in the first place, perhaps to keep knowledge of the Snarl secret, or because it's somehow necessary for them to completely destroy the old creation before they can make a new one. While it would certainly seem possible, I'm not convinced it's obviously true or unavoidable.

Especially since the very comic you linked to explicitly says "Nothing escaped it," which cuts this line of thought rather short.

[Dr.Zero]

Alternatively, maybe people did listen, but it’s been long enough that the few people who heard about the old world have long since passed on, and the stories about the previous world live on only as obscure myths that few people have heard.

This.
Also there aren't mass media around to spread the story: maybe there ARE still people around who met the survivors, or who know their story but the fact remained confined in whatever place that happened.