I can’t think of a situation where you would need to cache the AI, especially if you are handling things with checkpoints. Just reload, reset and rerun the AI routines. I suppose it could be useful if you had a group of bad guys that had evolved to a certain location, but even then id only cache their position, orientation, inventory ids (maybe ammo) and AI state. Nothing else is really needed.
I realize you were throwing out numbers but, in any situation where you are dealing with 2000+ entities you wont likely want to cache all of that. I could understand an argument being made for it, but really if you have that many physics driven entities its not likely to be made better with a save game. If they are AI driven that’s an entirely different problem, and one that is likely to run your users machine into the ground. Especially on mobile you are likely looking at a few dozen entities, on top of the typical controllers and such.
Try not to get too bogged down in any of this stuff. Make a game thats fun and see what you need instead of trying to over think the saving system.