Overpopulation is a problem. People will claim that the plant can support 20 billion+ people but they conveniently forget that these people will have an incredibly low standard of living.

Even if we were to assume that an arbitrarily low standard of living is acceptable, at some point that standard of living will include mass starvation and death so there is a real capacity limit. Being well below that limit is a virtue.

That I'm definitely not convinced of. With current levels of technology, poverty in a global setting has more to due with infrastructure and unrest than it does actual shortages. I suspect that we could pack on even more people with the industrialization of the third world, vastly increasing productivity.

More importantly, there's absolutely no reason to think that technology will somehow stop pushing the carrying capacity further. More people will yield more innovation, yielding more growth, yielding more people. If there's an endpoint to that, I doubt we'll see it any time soon. Yes, I too see how insane it is to think that sometime relatively soon global GDP will double twice in a year. But twice in a decade would be just as crazy 500 years ago.