Without digging to see if Phoenix is doing better than Las Vegas, your remark about Las Vegas not planning for the future is not borne out by reality. I recall from some home improvement TV show that the Las Vegas water authorities are paying people to remove lawns. This page
, this page
, and this page
bear that out and describe various other measures being taken. Forbidding new lawns, requiring removal of existing grassy areas nobody walks on, and paying people to remove existing lawns don't sound like measures someone blowing off the problem would be taking.
I don't know if that's enough for the size of the problem (I live in metro Chicago with a huge lake on our doorstep, so I lack perspective) but it doesn't support the implication that Las Vegas is sitting back and doing nothing.