The GAO will tell you the sky is falling if it will get them on the front page (they have, it did, here we are.) The fact is that the satellites are extremely reliable and it would be extraordinarily unlikely for enough to fail to cause performance problems. Sure, the satellites 'could' all start crashing into each other, getting dead batteries, or losing track of their cesium. The fact is that there have only been two failures in the history of US GPS, and they were at launch time and not in orbit.