They don't stop stronger women. Stronger women are obviously rarer, and as we know, just one woman of Cadsuane's strength has come in the last six hundred years.

Of course Sharina isn't representative of the majority of channelers out there! What is it in my model that makes you think any such thing?
A representative of the channeler out there is probably one of the older Novices who will definitely be Aes Sedai, but who isn't strong enough to make the Aes Sedai jump through hoops whooping in joy.

Who said it was deliberate? I said that it just so happens that the Tower has rarely seen someone stronger than Moiraine in the past thousand years. Doesn't mean women stronger than Moiraine are common. To the contrary, they keep getting rarer the farther up you move away from her.

The mechanism of plot convenience?

And that is exactly what I'm saying with my model too. If you put Cadsuane at strength 65, she is a pretty rare person. While the calculation is too difficult for me to carry out quickly, given the large number of channelers we're talking about (say 20,000 women distributed over a bell curve), I'd say that the difference in strength between Daigian and a woman of strength 50 would have to be less than one standard deviation to allow for 37.5% of women to be below her.
I'd argue that Caduane would lie in a similar position above 50, making her rare indeed. As rare as Daigian, in fact, who is uniquely weak in the weighted subset of the Tower, and still pretty rare in all the channeling population. Women like Morghase or Lanfear are further than even two standard deviations from the mean. 99% of the population falls withing 2 standard deviations, and Lanfear, Morghase, and some of the stronger Forsaken represent that last one percent, that vanishingly rare species of a very strong or very weak channeler.

Where do we know this from? For god's sake, Silviana, a woman weaker than Moiraine, can shield Egwene alone.
In battle, Egwene may well be able to tackle both Moiraine and Melaine. But they should be able to shield her easily.

I don't see why Egwene has to be stronger than Lanfear, sorry.