There are many ways to impart ones values onto society, children being only one of them. It's a very myopic view to think that your children will copy your values and/or culture without developing a mind of their own.
In any case, observing other couples was only one of many reasons we decided on this. We made the decision as well-informed adults and whether you think it "sad" or not is not all that important to us.
No, but if I ask my friends who have been to the restaurant about how it was, then I can glean enough information from their responses to gauge whether I want to go there as well. Even if they feel compelled by social mores to say it was all great, you can observe how they act in addition to the things they say. We decided that while no doubt others love their children very much, it wasn't a lifestyle we were interested in.
Why are parents so predisposed with trying to convince other people to have children as well, especially those who have made a decision not to, either way?
How are those analogous? People can live perfectly fine without a child but can't without a job (unless they live at home with their parents which coincidentally a lot of hikikomori do).
The ability to attract and successfully integrate hundreds of thousands of immigrants per year to support an oversized elderly population is not a luxury that most countries that can afford.
For most communities, normalizing childless will only come back to bite people in some form or another.
In any case, observing other couples was only one of many reasons we decided on this. We made the decision as well-informed adults and whether you think it "sad" or not is not all that important to us.