We could easily support our current population and quite a bit more if we actually respected and shared our resources responsibly.

I think this is a monumentally bad argument. Statistically, you are typing this from a computer or smartphone, with internet access, and are probably part of the segment of humanity that consumes the most resources (as am I). You are saying that if we want to have 15, 20, 30 billion people, we can all just live like the most impoverished people in the world and it will be fine.

It's a quality vs. quantity problem. Maximizing human numbers means we must have very low per-person consumption to minimize impact. Maximizing quality of life means we can all eat/travel/live/consume how we want and the environmental impact will still be low (but with fewer people). I think that's far more desirable.

EDIT: to distill down the argument further - you support voluntarily decreasing your standard of living, so that we can grow the population more, for the benefit of.... ? Creating lives that have not yet been born and can't care?

Whatever you want, I personally do not want a decrease in my standard of living, and especially not to feed the capitalist machine that requires growth at all costs. I presume, if faced with a real and present choice of the same kind, most people would choose to uphold their standard of living instead of sacrificing, only so that more billions of people can be born. When is it enough? Do we really NEED 8B+ people? I wouldn't mind living in a world of <1B people where everyone has luxury and plenty without destroying the planet in the process. We can't get there, but the principle still holds. Put resources into solving the problem of elder care, and just let the population decline naturally.

Hell, we could increase exponentially forever if we wanted. The universe is practically infinite

Yep, but it would take careful planning. Like, actually listen to the smart people who know how to design a long-term sustainable system, rather than just chasing profit at all costs.

It's not even that hard to do, just let people do the right thing. Every researcher that says "we could fix this", we automatically assume that voice will be ignored at best or crushed at worst, but it doesn't have to be this way.