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.
Now, maybe if the snowpack is gone and the rivers are dry someone could set up a gigantic solar desalination plant on the California coast and start producing water to be piped to Nevada and Arizona... which would need more gigantic solar plants to produce the electricity to power the pumps. Someone will have to pay for that, and the costs will be pretty high. Probably higher than the revenue from the tourism or agriculture in the region. What happens then? People leave the region.