Regarding population decline vs. extreme dexterity... there's surely a good bit of room to improve our ability to discover and foster the dexterous extremes. If "nurture" plays any fraction of a role in the process, there are too many paths through life in this world that would still deny someone the opportunity to achieve their potential dexterity. As a baseline, I'm just talking about abject poverty / famine / war and other horrors that would easily divert someone from even the opportunity to be spotted as a prodigy.

So even if world population declines for a while, I suspect it'd be possible to enjoy more mathematical talent in the coming decades / centuries.