> Our goal is to work, full time, as a professional pure mathematician; that naturally puts us in the labor market for pure mathematicians. We can't know that market exactly, of course[...]

For what it's worth, my classmates from college who have completed PhDs, based their postgrad career decisions on completely different factors – mostly their families and partners, and whether they're willing to move around (especially to rural areas) to target an extremely shrinking academic job pool.

EDIT: example that came to mind – I had a classmate who postdoc'd at Chicago, who decided to stay in town and work in finance rather than pursue some tenure-track offers at R1s, because his young one went to a prestigious UChicago Lab School and didn't want to uproot her.