Our water in the metro area mostly comes from West of the divide.
Printable View
Map of all the tunnels in Colorado moving water from west to east..
https://www.aspenjournalism.org/wp-c...PM-771x552.jpg
Map of water produced:
https://partingthewaters.files.wordp...ram_forweb.jpg
Phoenix also depends on massive amounts of electrical power (as does Las Vegas.) You think those cities would be as big as they are if air conditioning wasn't a thing? There's a reason that both Phoenix and LV were tiny before WWII - nobody in their right mind would live there in Summer.
Considering that most of the taxpayers in Colorado live on the Eastern slope, and that the tax revenues collected benefit the entire state - to include the Western slope - it's more accurate to say that Denver and the eastern slope BOUGHT the 6% of the Western slope's water that they are diverting.
Put another way, if you crunched the numbers I'll bet most Western Slope residents would be more than happy to trade the 6% of water (560,000 AF according to the chart posted above) that is diverted East in exchange for the tens or hundreds of millions of $$ they get in tax revenues from those of us on the Eastern slope. [Dunno]
ETA: In your imaginary situation where CO is divided into two states at the Continental Divide, what would happen? Well, the East would not be entitled to any water from the West but the West would not be entitled to any $$ from the East. So the East would be Water-poor and money-rich, and the West would be water-rich and money-poor. How long do you think that situation would last before the West decided to sell the one resource they have that the East needs? Either way you'd have a situation where money flows East to West and water flows West to East, which is the situation we have now.
ETA 2: And I haven't even touched on the impact of tourism from the Eastern slope to the Western. Millions - probably at least tens of millions if not hundreds - of $$ are earned by people on the Eastern slope and then spent on the Western slope. That's a direct benefit to the Western slope of people who live East of the divide.