> low birth rates are stigmatized as they are correlated with lower GDP

You have this backwards

the case in point was obviously Japan which fell from the third largest economy to the sixth largest in the past two decades.

Japan wants to be like China and the US, not like Qatar or Switzerland.

GDP is not GDP/Capita

I believe you have this backwards.

> I'm just demoralized that low birth rates are stigmatized as they are correlated with lower GDP

Low birth rates are correlated with higher GDP/capita; concerns are raised about them because they have a (decades delayed) effect of population aging producing a drop in the ratio of workers to those two old and infirm to work.

The real problem is "living behind your means" at a generational level by putting debt in future generations. This is a problem with democracy, which incentivises short term gains over longer term. The solution is democracies that aren't allowed to accumulate long term debt over agreed upon levels.

the hard part about shrinking populations is that they cause your population to get older which has a bunch of annoying economic side effects (like fewer people working)