Then the total hardness of the water will increase if the filter is not changed and all the equipment will calcify just as if you were using normal tap water. In addition, the germ count will increase...the water may even end up being worse than "normal" tap water...I got myself some aquarium test strips...with which you can check the water from the filter and change it if necessary....

The filter doesn't last forever either.

Nothing happens. You just have to descale a little more often.