I'll preface this by saying that I do quietly judge those who have many multiples of children (think 8+), but I also think that judgy comments towards those who already have kids is unproductive and makes you a bit of a twat. (It's not like you can put the kids back). I also don't agree with judging people for having more children than you when both parties have the amount of children they wanted. (Whether that be 0, 2, or 6). I think it's easier to justify what you are doing by looking at what others are doing that you're not. If you don't want children, you can say you've "earned" air travel, meat eating etc. At the end of the day, people have as many children as they choose to have. (Infertility notwithstanding). I may well be wrong, but I very much doubt there's many people out there who desperately want children but haven't had any because of environmental or population reasons only.
I've also seen the argument crop up that if those who care about the environment cease to have children, while those who don't keep having children, this will have a detrimental outcome long term. (Which is straying scarily close to eugenics and the "right" and "wrong" people having children territory, but still worth a passing thought).
However that doesn't invalidate the argument itself. Having children is a different kettle of fish to other things, as you're creating a whole new person with their own carbon footprint, possibly for the
next 80-100 years. If you live in a developed country, very few lifestyle changes could come close to compensating for that. Not only that, but the impact of having one child extends onto their descendants. Some might not have any children, some might have 5 or 6, but if you average it out to the 2.4 (less the .4 to make it easier and attempt to account for those lost in chidhood), 2 children in this generation will become 4 in the next, 8 in the one after that, and so on.
And supporting the aging population, whilst definitely a valid point which needs considering, also feels a bit like a Ponzi scheme to me - when the children who are born to support our elderly reach old age, they will need supporting, and so on, using each increasingly large generation to support the one before. It's also (again) putting short term human needs before what's best long term, for humans and the planet.
I think the other thing you're missing is that we all know people will continue to have children. You speak as if it's all or nothing, as if everyone is being advised to stop breeding. We know most people will continue having as many children as they want, so if just a few stop and think before having one, or one more, then that can only be a good thing.
I think overall, it's just something to bear in mind. We (collectively) do need to make some pretty drastic changes to our lifestyles and choices, and as a rule we (collectively) are not doing so, because we enjoy our lifestyles, freedoms and choices and so it's easier to label others as virtue-signalling or preachy than take action ourselves. I don't expect this to change any time soon (and I'll admit I'm not doing as much as I can, and need to make a much bigger effort). But our current lifestyles are not sustainable. Our current population level and growth rates are also not sustainable.
Having children is inherently selfish. We do it because of our biological urges, or because we want to, very rarely because we think we can benefit a child, but rather because a child will benefit us. What the environmental status does do is take away any claim that we have that reproducing is beneficial to society as a while. Yes, some people need to have children to keep humanity going, but more than enough people are already doing that. When we make a choice to have a child, we therefore do so knowing that that child will benefit only us and our immediate family, and be a detriment to the planet and environment. (There will be some exceptions, but on balance of probabilities our children will probably not cure cancer, or win a Nobel prize, etc).