If you follow the linked articles, the estimate of water shortage being an actual problem is on the timescale of roughly 80 years. That's more than enough time

If you follow the linked articles, the estimate of water shortage being an actual problem is on the timescale of roughly 80 years. That's more than enough time to figure out solutions or coordinate emigration to less arid places. So I guess we could say the current generations of people are not super screwed, but their kids will be pretty screwed if no one has taken action in ~50 years. From what I understand, the solid part of feces breaks down pretty quickly, but the bacteria and viruses that actually make people sick can remain a dangerous contaminant in water for some time. It ends up being a question of concentration - lots of water with only a few people = not much of a problem, but lots of people with not much water = big problem.