SF novels can be a way to explore the possible outcomes of decisions, technological changes, environmental change and more. "The Water Knife" is a well-informed book that covers a lot of ground. There are plenty of non-fiction books that cover the same on-the-ground truth, but don't bother to speculate on how things might evolve from here.

To your point, the book specifically floats a "vision" of a new kind of housing development in Las Vegas and Phoenix that the book calls "arcologies" but you might think of them as high-end earthships for the ultra-wealthy. They use very little outside water, endlessly internally recycling almost every drop of moisture within them. Accessible only to the ultra rich. I personally find this a much more believable idea and much less sci-fi than the notion that Nevada, Arizona etc. will receive water via a multi-thousand mile long, multi-thousand foot elevation spanning pipeline.

I think you may be underestimating the challenge of bringing water to the US southwest. Also, as I've mentioned several times in this thread, without commercial agriculture, there essentially would be no water issues here. When 77% of your water is being used to irrigate crops, most of which are exported from your region ... you've got a different kind of problem.