The point you raise is by far the strongest argument I know of against the idea.

However, it is a moral objection rather than a decision-theory objection. It sounds like you agree with me on the decision theory component of the idea: that if we were anthropically selfish, it would be rational for us to commit to making ancestor-simulations with afterlives. That's an interesting result in itself, isn't it? Let's go tell Ayn Rand.

When it comes to the morality of the idea, I might end up agreeing with you. We'll see. I think there are several minor considerations in favor of the proposal, and then this one massive consideration against it. Perhaps I'll make a post on it soon.