The biggest issue with population decline is generally an aging population. The older generations will require care, which we will not have the population to do.

The other issue is the ruling class is trying to replace good jobs with AI, keeping our wages for manual labor jobs low. Those end up bring harder to automate and have lower wages, so the incentive isn't there yet.

It turns out that when people are whinging about "population decline" it's actually a code word for "we're afraid people aren't having enough white babies." It's almost always a racist dogwhistle.