The (arguably) relevant metric is population change during a purported genocide, not afterward.
I think most people claiming a genocide is occurring are using a broad scope, like the conflict in Gaza since Oct 7 (if not something even broader), so it seems appropriate to look at the population change within that time period.
OTOH noone is claiming a genocide of Jews occurred from 1933-2025, so it wouldn't make sense to look at population change for that entire period.