I think you're making an awful lot of assumptions about people's attitudes post-Catacendre. You seem to think everyone would have abandoned their old traditions, prejudices and mores about who to have kids with, how many kids to have, etc. Because those traditions would have been founded under Final Empire ideals which are no longer relevant.
Yet, we know that almost 1/5 of the Originators were Terris, and that they still, 300+ years later tend to intermarry and live together in one small-ish community, semi-independent from the greater Basin. If anything, they seem to have leaned into their traditions harder to establish a sense of identity for themselves in the wake of the complete destruction of everything they'd ever known. Given the size of their group and the lack, according to Wax, of feruchemy extant in the population, Terris relationships likely involve few children in most cases and probably have for a long time.
How many of the people who survived the World of Ash were women of child-bearing age? We don't know. So the Lord Mistborn had a bunch of kids. Did other nobles follow suit? And did Spook's kids, who would have been important because they were his kids, marry similarly important people among the new nobility? Are birth records among the nobility (many of whom might have been allomancers, especially if Spook married his kids off in political marriages in order to keep the early government of the Basin stable) kept much more stringently than among the rest of the populace? This would explain why Wax can locate all these women and trace them back to Spook on readily-available geneological charts, if they were from important familiar with strong allomantic lines outside of just being related to the Lord Mistborn. And, logically, that's also where The Set would be looking for people who would be more likely to breed allomancers.
A member of a noble house that's been around for 300 years and intermarrying with other noble houses might have the same number of people between them and the Lord Mistborn as a canal worker who can also trace his roots back to Spook, but their potential for allomancy would be vastly different. Especially if the old noble houses also ended up marrying some of Spook's kids and several generations later someone can trace lines to Spook along multiple ancestors.