The GPS system was designed to be used for 25+- years, then replaced.

It hasn't been. I forget what year it was, 2000-2002 or thereabouts, it went past its design age. When the Satellites begin failing, breaking up or falling out of the sky, what then? Obviously, the Military has become completely dependent on the system, so something will be done. But, it was 15 yrs into the current system, before we were allowed decent accuracy for non-military receivers. Who is to say what will happen or be created as a replacement and when we will have access?