My feeling about the metaplot is it doesn't have to be immutable. There's a balance between allowing dynamic change that makes players feel that they are active agents for change versus providing an environment that's stable enough where they feel they have thematic elements they can depend upon and play off of. Like I feel superhero games focused on feature characters are the furthest extreme in the latter category since their primary draw is wanting to play recognizable characters, so changing them significantly undermines their appeal. On the other hand, original theme games benefit the most from dynamic change since it gives people a personal ownership over part of the world that they can point to and say they did that.
I think it should be extremely -hard- to change core aspects of theme, but I think for the stakes to feel real then they have to be changeable. For example, would I allow the big bads to destroy the main grid and central city of Arx and wipe out half the current PCs? Sure. Would I let that happen trivially and the result of a few mistakes? Of course not, and no one player is probably ever going to be able to bring that about something that would irrevocably shatter the stories of everyone else.