Is there a reason the Dark One could not or would not re-taint saidin ?
The conditions would have to be exactly right. Those conditions were set up while the seals were being placed on the Bore. The chances that exactly those conditions would occur again are fairly small. And that is saying that it was a volitional act rather than a side-effect of trying to stop the seals from being placed. After all, if the Dark One could taint saidin at will, why could he not taint saidar as well, and why would he not have done so?
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