> We simply don't need billions of humans.
You make an excellent point, and I'll raise a counterargument. I would claim, simply, that human life is intrinsically a good thing. I believe that my life is worth living, that it is better that I'm alive than if I were dead. I'll presume that everyone who reads this believes the same about their own lives. In that case, the more people alive, the better -- (if all else is equal).
That said, the philosopher Derek Parfit anticipated this and raises a counterargument to my argument above, known as the "mere addition paradox". [1] Roughly speaking, it goes as follows.
1. Suppose you have a population of 1,000,000 people, living happy, healthy, productive lives. Let's stipulate that this is a good thing.
2. Now, suppose in addition to this, suppose you have a second population of 1,000,000 people, who lead difficult lives with a lot of hardship, but whose lives are (perhaps barely) worth living. Then, if human life is intrisically good, then it is good that these people also exist. (This is the "mere addition" part.)
3. Given these two populations, equality is a good thing: lower the standard of living of the first million people ever so slightly, and raise the standard of the second group to match the first. This, too, seems to be a good thing.
4. Repeat. Eventually you arrive at a huge population, living lives of misery and squalor which are just slightly better than nothing -- but which one has argued is a better scenario than your original million people. This is the "repugnant conclusion".
[1] https://en.wikipedia.org/wiki/Mere_addition_paradox