In the long run, the problem is self-correcting
Leal mentioned agriculture as a major user of water. While it's true, agriculture is among the most efficient users of water. Because water is so scarce, and yet such a big part of agricultural production, farmers in the US learned decades ago how to minimize its use.
The one way agriculture can greatly reduce its water use is by not producing meat. And because the world population growth will increase by about 50% by 2050, that's the only way we will be able to feed them all. Instead of feeding four pounds of grain to produce one pound of meat, we will need to use that four pounds of grain to feed four people, and that will decrease overall water use by agriculture significantly. But even in the US get used to having meat only two or three times a week.
Also, there's no way the world can survive unless we come up with much cheaper energy. Wind and solar won't cut it without some new major technological improvement. Probably it will take developing fusion. So if we survive, we will have cheap enough energy to make desalination economically feasible.
