Because the longtermists are betting on 10^58 humans being alive one day. If you don't have children and population growth isn't exponential anymore their whole ideology comes crashing down.

If people voluntarily self limit the size of the population that is a good thing because it means we won't need to shoot each other if it turns out we are straining the earth too much.

You're asking the wrong guy; I don't care either way. :P

I'm just pointing out that in a wider discussion of "oh noes population decline oh noes low birth rates", it's not constructive to be talking about having 1 kid when there's a hard minimum of having 2 kids and ideally more.