> No human regards it as a tragedy that there are only 9 billion of us instead of 90 billion.

I have met some transhumanists and longtermists who would really like to see some orders of magnitude increase in the human population. Maybe they wouldn't say "tragedy", but they might say "burning imperative".

I also don't think it's clearly better for more beings to exist rather than fewer, but I just want to assure you that the full range of takes on population ethics definitely exists, and it's not simply a matter of straightforward common sense how many people (or horses) there ought to be.