Thank you for the insights!

Excuse me for repeating some of the things Brian said in reply to Calebp. Since I want to do a complete formulation of my arguments.

I think there are a few potential pushbacks to the "digital being dominating argument"

•  EMPIRICAL - Some digital beings might be nonhuman animals, or very similar to nonhuman animals in the relevant senses. And with that, we also need to consider the possibility that people's attitudes toward physical nonhuman animals might carry through to people's attitudes toward digital nonhuman animals. Some reasons people might simulate nonhuman animals:
  • Simulations for scientific studies
  • For education
  • For amusement/competitions
  • For aesthetics
  • For sadism
• EMPIRICAL - Brian mentioned that we might treat some digital people like we treat nonhuman animals. And one of the reasons could be that we make digital people like nonhuman animals. (And there seem to be incentives for this.) In this case, again, attitudes toward nonhuman animals now might matter a lot
• EMPIRICAL - How can we be sure that coming up with digital beings won't make use of physical animals? For instance, a Singaporean government-supported startup seems to be planning to sell semiconductors made from insects. 
• META/FIELD BUILDING - No longtermists seem to be ever trying to downplay the importance of future flesh and bone humans because they will be dominated by digital beings. In fact, they are advocating for them. So I wonder why there is a lot of willingness, if not eagerness, to downplay the importance of nonhuman animals in the long-term future using the digital people argument. (Yes, what happens to flesh and bone humans is likely to be relevant to what will happen to digital people. But the same goes for nonhuman animals too.)
• PHILOSOPHICAL - That there are even larger numbers doesn't make an astronomical number not astronomical. Then we need to think about certain philosophical questions, such as 
  • Can suffering and happiness actually be canceled out (as in both mentally, and morally)? 
  • Are above-0-experiences illusions? (there are people who argue that the feeling of happiness is not an actual positive state)
  • Whether we are justified in systematically leaving out proportionally small population groups, especially in cases when these groups are experiencing mostly extreme suffering.

I think we should have a lot of uncertainty about the future. For example:

1. There could be a high percentage of digital people but some non-digital people, and so animals still matter.

2. Digital people might cause suffering to digital animals.

3. We could treat digital people as terribly as we do animals.

Others have written about these ideas here: https://forum.effectivealtruism.org/topics/non-humans-and-the-long-term-future.

Thanks for your comment!