Basically, the social welfare function of the US citizenry is actually probably more cosmopolitan than current CBA. CBA is pretty well isolated from political scrutiny (most people have no idea what it is and it generally has bipartisan support), so I don’t think minor positive adjustments are a big risk.
I generally agree, which is why I explicitly acknowledge this in the post. But I also think you’re mistaken about what’s democratically feasible. The citizenry definitely gives a nonzero value to foreign lives (e.g., https://​​www.washingtonpost.com/​​news/​​monkey-cage/​​wp/​​2017/​​05/​​04/​​americans-love-to-hate-foreign-aid-but-the-right-argument-makes-them-like-it-a-lot-more/​​?noredirect=on&utm_term=.208535615b42), but current CBA only weighs them quantitatively (which is as good as not at all). I really doubt that diminishing the discounting rate by a point or so would engender political backlash; most people presumably think we should care about the future too and in any case have no clue what the discount rate is.
Basically, the social welfare function of the US citizenry is actually probably more cosmopolitan than current CBA. CBA is pretty well isolated from political scrutiny (most people have no idea what it is and it generally has bipartisan support), so I don’t think minor positive adjustments are a big risk.