I think the key may be that the seals represented an unbreakable physical object, that other people could use to symbolize that the Dark One was locked away.

As the belief that the seals are weakening spreads among the population, the seal itself starts to fade, because what kept the Dark One locked out of time in the first place was the lack of knowledge that an entity liked the Dark One existed.

There may have to be another Pattern Level Event, wherein Rand convinces everyone to forget the Dark One exists, perhaps by broadcasting the Aiel Singing, giving him a moment to heal the wound in reality that Lanfear had caused.