Yes! The problem is, women weaker than Daigian and women stronger than Elaida are very rare. The Tower has simply not gotten women showing at their doorstep who fall outside this range. They bemoan the loss of women stronger than Elaida, so we hear about it. No one bemoans to loss of Novices weaker than Sorelia, or women who can't even test for the shawl, so we're never told about it. The rare strength ranges at the top and bottom have gone down as a result of anemic recruitment. But we only hear about the loss of strong sisters.

Who said they're only prevented from finding weaker ones? Cadsuane said a thousand years ago, 50+ women would be stronger than Moiraine. So clearly stronger women aren't showing up. But what says weaker women aren't showing up either? How many women like Morghase show up in the Tower? How many like Sorelia, for that matter?

Here's an interesting quote:

This is Egwene describing an assembly of Wise Ones. This isn't all Wise Ones, by the way. Just a large sampling of them. And see the strength range. She's saying there are multiple women at the two extremes of the strength range, and the rest clearly fall in between. I think you'll find your average woman, at strength 50, somewhere in there. Women weaker and stronger than this range are so rare that an assembly of Wise Ones doesn't have any (though we know of several Wise Ones who are stronger, and there may well be several weaker, who are simply of no note because they can't contribute much). Sorelia is clearly not the bottom of the strength ladder. There's probably as much of a gulf between her and Moiraine after Ghenjei as there is between Amys and Lanfear.