Part of the theory I can't help but see the logic in. That- given the way this world works, there is so much suffering intrinsic to being alive for so many living things. So- wouldn't it be better if none of us lived to begin with? Perhaps- it's not like we'd be capable of knowing what we missed out on... We wouldn't exist!
The problem for me is the simple fact that we do exist. As soon as that happens, I think everyone should have the right of self determination. If they believe their life is worthwhile- I think it's wrong to start arguing with them that they'd be better off dead, along with everything else on this planet. Just the same as I think it's wrong for pro-lifers to insist that we stay alive only to suffer- when we don't want to. I don't agree with imposing your views on another person- in either direction. Especially when it comes to their life.
Of course, there's the hypothetical scenario of a red button that exterminates all life- or at least, all human life on this planet. Would it be more or less ethical to press it? I don't know- is the answer but I know I wouldn't be willing to make that decision for every living thing. That feels like God level thinking to me. So, I agree with certain principles behind promortalism, the same as I do antinatalism but I think ultimately, it's not for any of us to make decisions for other people.