1. People aren't having kids because they feel like it's utopia, quite the opposite.

2. Yes, it will be sad on one level to see a lot of empty houses etc, however, that means competition for housing will decrease and we will eventually be able to house everyone without concret-ing over more land.

3. Over consumption should not be the driving force of a civilization anymore, we've overconsumed for eons and now the chickens are coming home to roost in the form of climate change etc.

4. Overall it is not that bad to have fewer kids. Things will balance out and at some point the pendulum will swing back to people wanting more kids but currently they are too expensive and people are rightly too pessimistic to want to have them. Having large amount of children is a result of two things historically 1. High child mortality rate and an agrarian/early industrial society that needs a crap ton of bodies to perform tasks. 2. People having hope for the future and feeling secure enough to breed. (These are ofen oddly at odds with each other but it makes sense if you look at birth trends).

Currently we neither need large amounts of bodies nor do we have hope. Therefore, less kids.