The "fatigue" from long trips is hardly a result of having to keep in a lane.

It's more so the result of being awake, doing effectively nothing, for a long time. Lane Keep assistance is a useless technology for 99% of the population and the 1% who need it, likely shouldn't be driving a car anyways.

The more we "aid" fatigue, the longer drivers will attempt to drive. This cannot be a good outcome. The worst driving occurs when one is practically half asleep.