Just like there's a time value of money, there should be a time value of people. The unborn future billions should be discounted to the present value so that we can accurately compare them. What's the discount rate?
You're pushing the idea of the discount rate beyond its underlying assumptions. Short-term temporal discounting is premised on two concepts:
1) A pseudo-psychological principle that people value things more in the present than in the future;
2) An economic principle rooted in the assumption that there are alternative investments available for any given expenditure.
(1) is problematic because: a) it's not true at the scales we're talking about; and b) it's intrinsically tied to how someone existing in the present values future benefits. It takes a very narrow view of "utility" to argue that only the value to those in the present is relevant.
(2) is problematic because the assumption isn't necessarily true. Imagine a world where there are no investments that yield a consistent return such that an investment at time T0 yields exponentially increasing wealth at time T0+T. In such a world the second principle provides no reason to engage in temporal discounting. At the scales we're talking about, this second principle starts to break down. You can't assume that an investment in the present will continue to yield returns indefinitely into the future if your actions result in their being no future humans.
I think a more apropos basis for a guiding principle in this area is the observation that, in absolute terms, the productivity of society grows exponentially. A single person 1,000 years from now will produce much more, in absolute terms, than a single person today. If we define our metric more objectively, something as the sum of all production over the existence of humanity, then the proper course of action is the one that preserves as many future lives as possible, even at the expense of present lives, because most production will happen in the future.
It has to be very high, because our uncertainty about the future is absolutely enormous. I'd put the discount rate even higher than the monetary discount rate, which with fairly standard numbers is already effectively 0 in 20 years.
It takes a ridiculous discount rate to effectively become 0 in 20 years. 5%/year * 20 years = (1-.05)^20 = 35.8%. 10%/year * 20 years = (1-.1)^20 = 12.6%.
Still, 100 years is often considered a reasonable limit on such things as 5% * 100 years = 0.6%.
But note that most commenters there accept discounting based on risk, uncertainty, opportunity cost, etc. - which are exactly the grounds that most ordinary people, even in this very Hacker News page where you would think people would understand & accept things like expected value or probabilistic reasoning, claim to ignore the future based on.
A lot of those bases break down completely when you're talking about these time scales. Consider opportunity cost. It is not sensible to spend $50 to save $100 20 years in the future, because even at a fairly low rate of return the opportunity cost of that $50 now is over $100 in 20 years. Note that such thinking is unavoidably rooted in the idea that there will be a consistent rate of return over the period in question. That assumption isn't necessarily true if you're talking about the possibility of present decisions increasing the risk of wiping out humanity in the future.
If a similar pattern holds true, then the opportunity cost, in absolute terms, of a decision that results in fewer future humans is absolutely staggering.
