You're making the rather absurd assumption that because weaker women are coming to the Tower, there must be a worldwide drop in strength. RJ made several statements which joined together make it beyond doubt that the culling of male channelers did not reduce strength in the population, only the total number of channelers. Which makes sense. We know strength in the Power is not genetic at all. Nor does it depend on having the spark. Why would indiscriminate killing of all male sparkers result in only stronger channelers being born less frequently?