Garbage collection. If I haven't visited a chunk in a hundred levels, I'm sure to have forgotten everything about it (unless something special happened there, like a core mission). So, if I don't visit something for 100 levels, you can garbage collect the chunks in that region, and then leave them as nothing but a seed (as the cpu has time, spawn this to another thread when a player levels if you're an optimization junkie). This way, you can cut that linear rate significantly, as once you hit level 100, you can start culling regions the player hasn't been to in forever. You could even do it for settlements that are lost and forgotten, just keep what you need to generate it on disk. This dodges the re-gen + updates problem. Because if it generates something new, the player won't know, and probably won't care, especially if it preserves more continents for cross-continent interactions.
Lore-wise, you can chalk it up to things coming into that multiverse flux you already have with multiplayer. I don't know how specifics would work, as I'm still rather fuzzy on AVWW lore.
I dislike the idea of losing entire continents or adding mini-games/continent shrinkage too. But, this won't solve the problem, just extend it. I could go with eventually losing really really old continents, but they would have to be so old I don't even remember that they exist because they are only seen at the bottom of long lists. I would prefer something like Hearteater's suggestion of a type of map that includes those old continents you lose, but in a vague manner.