Cadsuane tells us that when she was younger, there were more women of her strength level, but by NS, they'd begun to die off (or were killed). She says that there are a few women who are close to her in the "modern" generation, but they are few and the women below them are much more common.

This points to the general decline of the strength of Aes Sedai in the White Tower. We know that channelers are not growing weaker in general, but the Tower is for some reason.

I think that the culling theory makes sense, but not from the angle the Aes Sedai are thinking of it. They claim removing men from the gene pool has caused the decline in strength. I think, rather, that by removing a significant portion of the channeling women from the same areas generation after generation, and not adding to the subsequent generations by reproducing, the Aes Sedai have weakened the pool of potential candidates.

This is not to say that I agree that the curve should be set at 50%... I just wanted to point out a mechanism that the Tower is using that reduces the strength of their sisters in general.