Some believe that people not dying like flies is a good thing.
You forgot that lower fertility rate means fewer kids in school, college, etc. Thus the total percentage of workers vs non-workers is not influenced as you might believe. It is also to a large extent a problem with statistics - we look at GDP growth - and with fewer people GDP will decrease, if GDP per capita stays the same.
However, US is already at total fertility rate 1.87 - below the UK, and clearly below replacement fertility rate in the US.