It was alluded to by the visions regarding the takeover of technology (via Mat and the Cairhien research institute). Technology is more accessible than channeling and makes it redundant in many ways (for example, cannons outrange channelers).

RJ also said that if both powers were used to seal the Bore, both would be Tainted, but Rand seems to be working towards the conclusion that he needs to use both to seal the Bore. That's the "obvious" solution to him, and therefore the most likely outcome at present. If he continues with the current plan, the Taint will affect both side of the power and the only way another Breaking would be averted is to block of the power for good. This would actually solve a heap of problems, imo, because the power does very little good for anyone right now (it is primarily used a a weapon or to manipulate people). Take away the power and suddenly you remove part of the ethical dilemma of Seanchan rule, as well as the meddling and plotting of the White Tower.