Do humans without lawns consume that much water? Have you worked on something where this problem came up in some unexpected way? Yes pipes need to be updated if a lot of tall complexes are built, but is total water that much at all?
I asked Claude to do the math, and it claimed reducing alfalfa use (not all use, just alfalfa) by the Colorado river by 1% per 1 million people who move from a place not already supplied by it.
I think when it comes to water a really important mental model everyone needs to have is that a human without a lawn or pool uses really tiny amounts of water at scale directly.