You can take the Hari Seldon approach and abstract away the macro level 'crowds/nations' and simulate them when they're not in the observer's sphere of influence/vision using a (much) higher level of abstraction.
To take a semi-long term example. If you go carve your name in a tree, when you come back 50 years later it better show your name distorted over those 50 years. If you cut open the tree. It better show a record of every weather event over those 50 years. Every piece of the world you inspect will need the simulation run and not just for it but for all the things that affected it and all the things that affected the things that affected it etc etc recursively until you've simulated every in the universe.
Minecraft is an example of the latter: if you leave a chunk, mobs despawn and decay and growth stops.
But a simulation need not work that way. E.g. instead of continuing to run a simulation, it can simulate macro-indocators, and continue to simulate micro-details with a strong bias towards reverting to a prediction from the macro-indicators, and each element that gets there can be discarded.
To go back to the Minecraft example, you set a "decay rate" for mobs that reflects a believable rate of dispersal, or simulate changes to vegetation towards what your generating function will produce until they match.
Then when the player re-enters at t+a_lot, you just run your generating function where time is one parameter, and apply whatever reduced delta is still relevant.
Yes, you'd still need to simulate a lot, but you'd be able to prune most things.
So the question is meaningless without defining your requirements.