Socialism would no doubt increase the planets carrying capacity for humans, but not make it limitless. It is also nowhere near close to being implemented so I am assessing the world that we have, not the one I’d like it to be. Also, even if we did away with capitalism tomorrow we’d probably still need to discuss reasonable population growth and come up with a reasonable estimate for our planet’s carrying capacity which could be weighed against quality of life, human happiness, etc as we transition our economy away from late stage capitalism.
I wouldn’t. Really all post industrial countries need to do is stop trying to directly insentivize having kids and maybe provide access to free/low cost contraceptives. I think that’s a lot easier than having socialism implemented in enough countries for it to matter.