You write that:
(A) “We are profoundly uncertain about whether most animals’ lives are dominated by pleasure or suffering, or even how to go about weighing these up. Therefore, it may be prudent to concentrate on a measure of “relative welfare expectancy” (RWE), representing the normalized welfare expectancy of a population divided by its life expectancy.”
But you also write that:
(B) “A plausible working hypothesis, however, is that the average welfare experienced by an animal of a given age is proportional to their probability of surviving that period of life.”
Unfortunately, these views seem inconsistent. The (A) suggests that we should avoid making assumptions about whether increasing wild animal lifetimes is good or bad for the animals, while the (B) tells us to assume that welfare at a given age depends upon survivorship. However, high survivorship corrosponds to high lifetimes, so these are effectively the same assumptions.
You might defend your position by saying that welfare at each age is very small in expectation, so the expected value of increasing animal lifetimes, while holding welfare at each age constant, is neglible. However, this argument makes a significant assumption about which probability distribution over welfare at each age would be rational. Thus, it doesn’t square well with your motivation behind ignoring lifetimes.
I think this would be way easier to understand with an equation or two. Let w be overall lifetime wellbeing, let wt be age-specific wellbeing at time t, let L be lifetime and let us denote averages over lifetime by an overbar. If so, it seems like the “normalized age-specific wellfare” is wt,norm=wt/¯w. It is not clear what “this normalized welfare expectancy” refers to, since it can either mean wt,norm or wnorm=∑twt,norm (I assume here that overall wellbeing is the sum of age-specific wellbeing). Thus, the RWE is calculated as follows:
I find both of these formulas to be rather strange, and devoid of a rationale. Have I misunderstood you?