I understand the point your making, but what if the simulation was actually not shared at all?
Perhaps in this scenario the human brain is the only required hardware? Then there would only be one "base simulation" that is in fact just a basic set of prompts, rules, and initial visual stimulus that is then sent to each person in essence creating a whole separate simulation within each individual. Everything that happens after that is created based on how each individual reacts to the initial prompts. The main system would not have to create any new data to keep the simulation growing because the human mind would create and store all new information within itself. Each new person born would have all the additional hard drive and processing power needed to keep the simulation going for the rest of their lives.
Just consider that if the world as we know it is just a simulation, and that simulation is all we have ever known since birth, how would you ever know if the other people are real or not? Would it even matter?
It looks like a portal to Narnia lol