if you take future generations into account, a 1% reduction in existential risk could save 10^32 lives.

And if you don't solve aging ever, and take future generations into account, then 100% of all human beings ever born will die, which kills at least as many people as any existential risk. (The inevitability of solving aging given enough time but no particular will or effort towards it isn't obvious - so it's not just a matter of saving a few decades worth of 56 million lives a year.)