If we were to build a self-replicating spacecraft, our estimates of existential risks would go down some. But the risks themselves would be unaffected.

To take an extreme example, building a self-replicating spacecraft, copying it off a few million times, and sending people to other galaxies would, if successful, reduce existential risks. I agree that merely making theoretical arguments constitutes making maps, not changing the territory. I also tentatively agree that just building a prototype spacecraft and not actually using it probably won't reduce existential risk.