Their findings weren't implemented.
What's going to change our behavior?
Implement their findings.
Where would our economy and standard of living be if we had 50M fewer consumers/workers?
Better off, if those 50M consumed more than they produced - as there's every reason to believe they did, since they were not admitted on the basis of valuable skills.
Even the authors admit that their prescription would have a minimal impact on fertility rate:
"Although the elasticity of the birthrate with respect to the exemption is not large, it appears that the US can influence to a degree the fertility decisions of its citizens through deliberate changes in tax policies."
In fact, the inflation adjusted personal exemption did increase from 1990 to 2017 but the birthrate continued to decline.