The price of water has gone up for a multitude of factors. One of them is water savings in general, but not primarily because the sewage system requires regular flushes. The reason is that water gets paid per qubic meter and includes a fresh water and a waster water component. The assumption is that almost all fresh water you use ends up as waste water. Now, the grid has a very substantial fixed-price component that's largely independent from the actual current volume being used. Putting pipes in the ground and maintaining them there is an actual costly endeavour. If water use now drops, and the baseline cost remains stable, then it's entirely expected that the price per volume rises. It's simple math. The same baseline cost needs to be brought in via less volume.

This will also happen to people that use residential gas. As less and less people use residential gas, the maintenance of the gas network gets distributed among less and less customers.

> The 'balcony power stations' are the same thing. They get subsidised, and you even get a fixed kWh price when pushing into the grid.

They are subsidized on purchase, but the price they get when pushing energy into the grid is by default fixed at 0. The network accepts the power, but there's no payment. It's also capped at 800W delivery, meaning that at peak power generation, you'd earn a whopping 5 cent an hour with the current subsidy for full scale solar power. So in practice, the only benefit owners have is that they draw less power from the net which is much more attractive because of the pricing structure. You can, optionally, register your balcony power station as a regular solar power plant, but then you're subject to a whole bunch of rules and regulations (for example you need a suitable elctricity meter etc.). This option is generally not attractive for such small power generations.

Fundamentally, though, the same issue as with the water and gas network exists with all localized (solar) power generation. If more and more people use the grid only as a backup, or for winter energy needs, then the overhead of maintaining the grid will have a larger cost contribution to the total cost of electricity.