The game barely manages to simulate as it is with its current design. The agent approach simply didn't scale up to the sizes of maps and population that would be necessary. I doubt it had anything to do with memory limits, looks like a CPU performance issue to me that kicked in much earlier than they expected. It looks like they also expected to be able to multi thread with an agents approach but they never wrote it as such and they ended up not having much multi threading at all. Technically the game was just poorly designed and written, this shows in the level of bugs, in the technical issues with the game and the performance limits it has.