Agreed that a blanket US based IFR applied globally gives only a ballpark figured and more accurate numbers could be gleaned through increased cohort stratification by risk factors. But for a quick back-of-the-napkin number, it provides a range that’s pretty close to what you’d come up with when doing a whole lot more work. Western Europe sees similar IFR numbers to the US. Latin/South America, Asia, and Sub-Saharan Africa are the larger question marks, but it’s not like they’re going to see 10% IFR or 0.00001% IFR suddenly. They’re still humans. So if the IFR for Africa shifts to between 0.5% and 0.85% and the IFR for Asia shifts to 0.7% - 1.1%, we’re still talking about very similar numbers.
Think AGE, Einstain.
Lots of the developing world (== higher birth rates) have far lower % of population over age 70, which is where most of the deaths happen; as well as far less, hmm, diabetes (no chugging of high fructose corn syrup 24x7).