We are consuming water from places where it has been relatively static, at rates far beyond its natural replenishment rate. THAT'S the issue. Yes, this is a closed system, and the human circulatory system is a closed system. But it's bad if you drain the blood from your brain faster than it is replenished.
It's like saying military burn pits are okay because there may be medical advancements in the future that can be used to treat afflicted soldiers.
I do agree though, if science wasn't verifiable, it would sound like a pyramid scheme.
If you showed today's world to an optimist from a century ago they would see many wondrous things they would never have thought of, but they would also fail to see many of their predicted advancements. And I do think that what optimists predict is usually far more impressive than what actually comes to fruition. That's not to say the pessimists are right, but the optimists simply don't have the track record you seem to think they have.
It is entirely possible with our current level of technology to adjust to the realities of the physical limitations in the world as we know it, without needing to resort to picking “who lives and dies”.
There's a huge difference of usability between different water sources. You can't irrigate your orchards with salt water from the ocean. You can't bathe in waste water from most manufacturing plants. You can't drink ground water that has been contaminated by fracking.
There is a finite amount of potable water. There is a finite amount of usable water for agriculture. Pointing at all of the water on earth and acting as if it's all immediately usable without significant, costly barriers is silly.
The issue isn’t that the water will cease to exist, or that the planet will literally be destroyed. It is that our way of life is unsustainable and actively degrades the future quality of life for us in the future (and our children).
You can compare our mindless pumping of water from the ground to the ignorant farming practices that lead to the dust bowl. If we don’t use our knowledge to take steps to avoid running out of usable water by using it less and more intelligently now, we will come to a hard point in the future.
Which takes cooperation and empathy for everyone’s situations and the overall reality in front of us.
Water is much stronger than that. It just passes through our processes and will outlast us all.