Do you not argue for that at all? I thought you were going in the direction of establishing an axiological and deontic parallelism between the "wretched child" and the "happy child".

I do some of that in chapter 4. I don't engage with speculative arguments that the future will be bad (e.g. the dystopian scenarios that negative utilitarians like to discuss) or make my case by appealing to positive trends of the sort discussed by Pinker in Better Angels. Carl Shulman and I are putting together some thoughts on some of these issues at the moment.

The quoted passage ("all potential value is found in [the existence of] the well-being of the astronomical numbers of people who could populate the far future") strongly suggests a classical total population ethics, which is rejected by negative utilitarianism and person-affecting views. And the "therefore" suggests that the crucial issue here is time preference, which is a popular and incorrect perception.

Maybe so. I think the key is how you interpret the word "value." If you interpret as "only positive value" then negative utilitarians disagree but only because they think there isn't any possible positive value. If you interpret it as "positive or negative value" I think they should agree for pretty straightforward reasons.