Fury Road isn't that bad an ending. Water flows, but we don't know how much of a shortage there is nor if will be that much of a problem. This can also be a political thing for Furiosa to gain the good will of the populace before instituting a better rationing system.
But yeah, the end of Logan's Run is far worse. You have a people who have lived in an authoritarian society where most things are provided for to being forced to requiring free agency in order to live in a frontier like setting they aren't used to. Most of the people are going to die.