Well, yes, but we have to weight according to the chances of them actually being run. Take movies, for instance. A non-trivial fraction of them are simulations of the past - since we are interested in the past.

As far as I can tell, your argument is that the proportion of simulations of us that are run by our descendents is significant with respect to the total number of simulations of us, because the number of simulations we currently run of our past is significant with respect to the total number of simulations of our past.

I don't think I ever said that. Rather, I was criticising the idea of comparing the number of past simulations with the number of "possible simulations" - by pointing out that considering the number of possible simulations ignored the important issue of motive - and I gave an example illustrating how motive might matter.

Obvious alternatives to ancestor simulations include optimisationverse and the adapted universe. We do have a whole universe worth's of data about which idea is more likely.