But then this would not be an ancestor simulation and they would not be our future. It’s difficult to change the laws of physics and still have a universe that resembles ours in any way. Invoking “maybe there’s some other universe with totally different constraints than ours that is simulating us” is an unfalsifiable speculation which we have no reason to believe in whatever. Bostrom would know this which is why he tries to construct a somewhat more robust argument than that. My postscript gives more detailed arguments rebutting it than its mere technical implausibility.