Historical population declines worse than the current ones like the Black Plague and WW2 have always resulted in a better world for everyone.

You do have to consider how those population declines happened though and in what state the world was in during and after. These countries with lower birthrates aren’t war-torn countries or cesspits. Birthrates dropped because people could make a decision not to have children. They are more educated in general than the cases you stated and willingly (well, forced by economic pressure) making conscious decisions not to make babies.

When population goes back to 1950’s levels, labor will be valued like it was in the 1950’s, and the average person will once again have the wealth and leisure time to have a family.

I like your optimism.