There are huge problems with declining birth rates. Mostly due to the demographic mismatch of a large number of old people being supported by a small number of young people.

You might say that's a temporary problem. Eventually all the old people will die and a new normal will be established. However, a country's infrastructure works at scale and doesn't become more efficient or cheaper with less people. Quite the opposite. Fewer people means a complete redesign of infrastructure.

Fewer people also means less chances of someone doing something groundbreaking. Problems like climate change need innovative solutions. A decline in birthrates isn't going to fix it, but fewer people overall means fewer people working on these types of problems, meaning it'll take longer to solve. We're already on a time crunch for these things.

Most people would have more children if they could. Birthrates are declining due to outside factors like increases in cost of living and difficulties associated with child care. We can't force people to have kids to fix declining birth rates, but we can fix issues preventing people from having the children they want.