I am happy to grant, from a Buddhist perspective, that the sum total of human welfare, from the beginning of our species through the present, has been negative. I am even willing to suppose, again from a Buddhist perspective, that total human welfare in the present is negative.

Ah, I'm so relieved we can reject pessimism! ;)

I read the paper, then asked Claude 3 to summarise. I endorse the following summary as accurate:

The key argument in this paper is that Buddhism is not necessarily committed to pessimistic views about the value of unawakened human lives. Specifically, the author argues against two possible pessimistic Buddhist positions:

1. The Pessimistic Assumption - the view that any mental state characterized by dukkha (dissatisfaction, unease, suffering) is on balance bad. 
2. The Pessimistic Conclusion - the view that over the course of an unawakened life, dukkha will always outweigh the good.

Against the Pessimistic Assumption, the author contends that even if an experience contains some element of dukkha, it can still be good overall if its positive features outweigh the negative ones. The mere presence of dukkha does not make an experience bad on balance.

Against the Pessimistic Conclusion, the author argues that once we reject the Pessimistic Assumption and allow that some discrete unawakened experiences can be good overall, then given the right causes and conditions, it's possible for a whole unawakened life to be good on balance, even if it necessarily contains a nontrivial amount of dukkha.

The core claim is that while Buddhism regards dukkha as pervasive in unawakened existence, this is compatible with unawakened lives sometimes being good overall. Dukkha is an inevitable bad-making feature, but other good-making features can potentially outweigh it. Thus, Buddhism need not be committed to a pessimistic assessment of unawakened human lives.

Thanks for this. 

I take the central claim to be:

Even if an experience contains some element of dukkha, it can still be good overall if its positive features outweigh the negative ones. The mere presence of dukkha does not make an experience bad on balance.

I agree with this, and also agree that it's often overlooked.