The vast majority of Westlands Sparkers either become AS, wind up in the Kin or else they simply die of the sickness. There is a small portion that become Wilders, but even these women seem to be somewhat stigmatized in the Westlands. Faile notes it when she discovers Morgase/Maighdin is a Wilder, and Moiraine says something similar in New Spring when she is masquerading as a Wilder.

In Seanchan they also find and cultivate every single woman who can learn ... aka Sul'dam ... who we have no reason to believe abstain from having children or getting married etc... thus keeping the genetic trait active, much as in the Westlands, the non-sparkers are keeping the trait alive.

The real mystery is why haven't the Seanchan noticed that Sul'dam are much more likely to have children and/or siblings who are Sul'dam or damane. Probably because damane are stricken from the family tree, but you'd think someone would notice how many siblings/cousins/relatives are interlinked to the Sul'dam and damane.

Looking at WO and WF it's clear that family lines such as Talaana's are noted as breeding true, this should be just as true in Seanchan if the Sul'dam are having kids, and noticeable from looking at the families who have many members taken as damane or Sul'dam.