Earlier quoted context omitted.
So... let them use up the water so that it's affordable to stop them from using up the water?
The goal isn't to "stop them from using up the water", it's to charge them the true price of providing that water, and charging a bit extra so we can subsidize the price of water for the poor.
However, when the resource is no longer plentiful, it is unwise to try the "let the market fix it" route (always a popular one on HN), because the resource in question can be consumed much faster than the systems you are describing can combat it (or can even be gotten off the ground.)