A Quick Account Of Why Temporal Discounting Makes No Sense
At least, intrinsically
A temporal discount rate involves holding that the further in time, the less important things are. For example, holding that something which would kill 100 people gets 1% less bad, for each year delay. Discounting makes some sense on instrumental grounds. After all, given uncertainty, we can’t be that confident about the far future, so the further in time we project, the less weight we should give towards our projections.
However, discounting intrinsically makes no sense, as we’ll see, despite the frequent fondness of economists for discounting. Suppose your discount rate is only 1%—a very modest rate. Well, on this view, one person being killed in the year zero would be as bad as 546786506.396 people being killed in the year 2022. This is clearly ludicrous.
I think the argument is missing the fact that you don't use temporal discounting to look at the past. Temporal discounting is about uncertainty--you're making a choice and you're uncertain about the future. The further in the future, the more uncertain it is. But if you look backwards, there is no uncertainty (or at least a lot less). To give an example, if you were Hitler's nanny, deciding to smother him would have no apparent value (probably negative value, at least according to his family). It would be very difficult to predict what his effect over the next 60 years would be. If you were a time traveler, and knew about what Hitler was going to do, then the decision to smother him might have far more value than deciding the death of one random person today. There would still be uncertainty, because you don't know what would have happened if you started history forward from that same point. So, you're right: there is no obvious reason to believe killing one person in the year zero is worse than killing one person today, because at their respective times, the decisions were equivalent. But you can still use temporal discounting to look at the future, because the chances of your choices today having the favorable outcome you anticipate are diminished the further in the future they are. Another way to look at it: temporal discounting would not have any value, if you knew the future (because again, there would be no uncertainty).
Hyperbolic discounting (rather than exponential) can avoid this implication. The few contemporary philosophers sympathetic to discounting would support hyperbolic over exponential discounting.