What is this supposed to mean? You flush less water, therefore water price is more expensive, because flushed water needs to be paid too?
I honestly don't see the problem, it's probably still worth it (because society still needs to provide less tap water and saves there).
If you, a single person, cut your water usage in half, you pay half as much. But if everybody uses half as much, the system still needs about the same amount of funding. So now you double the per-unit price, and everybody pays the same they were before spending money on water saving features. In this case, even if each person used half as much water, the total water needed isn't cut in half because the sewers need more water to function.
(Also, water isn't "used"; most of it's transported, cleaned, transported, dirtied, cleaned again, transported)
Or not. https://www.welt.de/wirtschaft/article152318777/Wassersparen...
Edit: parent changed his answer, I have included it now.