So even if the US satellites all fall out of the sky, there will be another system eventually (requiring new receivers) available to the world.

Supposedly the European Galileo system will be a subscription system. Time will tell.

As far as the US system, Navstar, there are currently 31 sats in space, I believe 24 is all it takes to achieve full operational status. There are 4 sats that are off line for either maintenance or other reasons, and the last sat launched was Aug 2009... so the system is being maintained.

As far as celestial nav, I just want to learn it... both as a backup and to increase my knowledge of "how things work."

I'll probably get a plastic sextant just to "play" with.