I believe the final revelation Rand has is that The Dark One isn't the source of evil within the world, merely a power source that evil people tap to facilitate their own ends.

Thus, expunging it by bringing it into the Pattern carried huge risks/costs while doing very little to actually discourage future calamities.

In the end, Shai'tan is an alien and unknowable thing. But its not an actual source of humanity's problems. Just a vector for them.