Legitimate question: what is the concern with declining birth rate? Im no expert but it seems like the world currently has way more people than it ever had before and even in specific developed nations those nations were arguably doing just fine 50 years ago when they had lower populations too. What is the risk here?
If you are set on not voting in the presidential election, I would like to encourage you to still vote (if you didn’t already intend to) and just not vote on that question on the ballot, for two reaons:
Local ballot measures and politicians are still quite relevant and your vote is still fairly impactful there relative to the federal election (depending on your state) and
it sends a significantly stronger message to both parties that they are putting forward garbage candidates to vote and just abstain from the one question. Not voting is easy to mislabel as voter suppression by the opposition or any number of other misleading causes when voting, just not for Biden or trump is much more clear what you are mad about.