> any assumed meaning of our existence only matters while we're alive.

What do you mean by this? It is obviously possible to have an impact that lasts after you die and that people view as meaningful long after you die.

If the meaning of your life is to raise your kids well, that still matters after you die. Or if you invented calculus, or general relativity, or conquered Egypt and Persia, or wrote an epic poem read for thousands of years.

The world will go on without you, but it will be different, and maybe meaningfully so to those left.