The emphasized part is you moving the goalposts. If you want to assert that already low-risk individuals can further reduce their death rate using the vaccine (undoubtedly true), then it's entirely fair to ask whether doing so creates unknown, long-term risks.
For your younger people--who are unlikely to die from either COVID or the vaccine--the long-tail risks of a brand new category of vaccine are meaningful. My kids shrugged off COVID (before the vaccine was available) in half a day. We gave them the vaccines anyway, because we're sheeple. Hopefully, these new mRNA vaccines behave just like traditional vaccines and don't create any additional long-term risks. But let's not pretend we have decades of experience with these vaccines to prove that fact.