The bubble answer is logical on the surface, but gonzo when you look deeper. Why should a quest in Heavensward take place in the same span of time as a quest in ARR? Sequentially, one happens after the other. Saying B happens after A does not change the position or shape of A, not even if you're just learning how to read.

Having Shadowbringers take place eight years on shouldn't have any affect at all on past quests, or change them. Quests from the old expansions can be treated as having happened in the past. Players moving through those quests are moving forward through time, playing out events as they happened, and eliding over the necessary time skips.

If time can be a bubble, it can also be a foggy gray expanse inside that bubble. And bubbles that contain more mass do tend to get larger.