If you are stumbling along, and experience a depressurization, then you swing into the emergency descent - which as a standard stops at FL100. It would be all too easy to do this over the Alps - with a nasty TERRAIN alert and subsequent problems. Hence the brief that, once over terrain with a LSALT above 10,000 we need to be aware that we shall do something different.
We use 14,000 as our initial descent target, assuming flat terrain. Before we start down, however, we'll know how far we want to go. If it's a long way and we're shooting for an equal time point alternate (ETP alternate), then we have other altitudes we'll use.
10,000 is a rather arbitrary altitude, which appears to presume no oxygen available, and a short cruise to where ever one intends to land. Once one gets down to lower altitudes, one had better have a good plan and a short flight.
Not these days, with dual GPS enabled P-RNAV capable FMCs.
That's what the crew in American Airlines #965 thought, too. Lots of redundancy, a solid data base, enter the information, and you're good to go. At least a little way.
We have seven GPS sources on board, and triple INS/IRS, and navigation is always a primary concern. Particularly when operating adjacent to hostile airspace and unfriendly countries.
Your FMC won't tell you about avoiding the terrain on the way down, save for EGPWS integration, and that's nothing to navigate by. It's an emergency device, and if you're receiving warnings from that, then you're really in trouble. You should know where you are and how low you can go and that's a function of navigation, not simply pushing buttons on the FMC.
Too many "children of the magenta line" today that think they have a FMS/FMC, and they're golden. The crew of Flight 965 was golden, too. Now, of course, they're dead.