DeepBlueJoy
Lion Faced Kitteh
I figure that young women coming to puberty would become slayers after the 'big slayer' activation spell. Those with the gene who were too young to be activated by the spell would still exist. And there's nothing that says that ONLY potentials can make more potentials. There's no reason to believe that Joyce was a potential.In a roleplaying game I actually went the opposite way. How do you get potential slayers? They are born from other potential slayers. Activating all the potentials meant you effectively ended the Slayer line, and the party had to fix that.
I think there are probably many families out there with the slayer genes and once they come of age, more slayers will be born.
Also, there's no reason for the living slayers to not have children. Nikki had a child (Robin). There's absolutely nothing to stop slayers from having families. Now that there are so many of them, they don't all have to be fighting all the time. Soldiers in the military have kids all the time.