Estimate 3,000,000,000 metric tons of human waste generated every year by 7 billion people.
Estimate 1,500 tons of water to flush each ton of "waste."
Estimate 1,260,000,000,000,000,000,000 liters of water on earth.
Estimate .396% of water is available in all rivers, lakes, aquifers, etc. That makes 4,989,600,000,000,000,000 liters available to us.
Estimate all that water at 1 kilogram per liter, and we know 1000 kilograms per metric ton. That means 4,989,600,000,000,000 tons of water available to us.
To flush all the excretia from the humans on earth, we need 4,500,000,000,000 tons of water. That leaves us with 49,8510,000,000,000 tons. Then subtract how much we need for industry at 752,000,000,000 tons. We need 2,000,000,000,000 tons for agriculture (not counting rain.) To be able to flush all the poo, we'd need more than twice as much as all the water we use to grow food.
That leaves us with 495,758,000,000,000. The amount of water actually encapsulated in all humans is about 318,500,000 tons. Anyway, this could to on and on until I theoretically run out of water.
The point is, there's not really all that much water around, especially when humans are already using more than 54% of the water available, and that's now. This number is expected to grow to 70% by 2025. Even if the money was available to convert all the world to water based sanitation, it couldn't be done. Consider the 1-2 billion people with no fresh water or sanitation. That number will grow, not decrease. There's just not enough water out there.