There are no guarantees that Van will continue to have wet winters forever, either. The US Southwest (Arizona) and West Coast of south America used to support agrarian societies, but no longer do (this was long before modern climate change).
The problem about being "water rich", is that it makes us too lazy and complacent (like selling it for $2.50 per million litres) to treat potable water as a resource and implement measures for conservation and lowering waste.
Nothing is forever, but don't the models show us having drier summers and wetter winters. It certainly seems to be the case.
IMO, if the PNW runs dry we are all fooked anyway. In the meantime our RE values will stay high.
Also, maybe the Anasazi were just pussies and couldn't handle a little dryness.