Q) Should I be worried?
A) I don't think so.
So you would think, but this isn't about total failure, it's about gaps appearing in less-travelled parts of the world due to, as SD points out, the number of functioning satellites falling below the 24 needed for full coverage.
It's not that it might not be there, but it might not be there if you need it in mist at night near the sketchier sort of reef.
I'm not trying to be alarmist here, but simply realistic that GPS is a system reliant on a certain minimum number of functioning elements. Reduce or remove those elements, and coverage is also reduced along with accuracy. Three satellites is good (but not if they are also at 20 degrees above the horizon), but more locks are better, because of this very interconnectivity of the signal processing.
The way people talk about GPS, it's as permanent as the stars and as reliable as the sunrise. Actually, it's more like the electricity grid: 99% fine, but parts of it are pretty old and prone to failure, which is why you keep candles and flashlights in a drawer. Also, to extend the analogy, if the GPS constellation is missing a few functioning satellites, does this impair accuracy? Would the average sailor be aware if his "circle of confidence" went from five metres to five hundred because there were just 21 satellites and not 24 working? A subtle or intermittent degradation is in some ways worse than a straight outage.
Even if they did, how many voyagers today have such "candles and flashlights" aboard and can use them? Half the people here can't heave to.
My wife's first CN class is this evening...purely coincidental, I assure you.