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.
It is an arbitrary altitude - you'll have to talk to the regulators about why they chose it. It is easier to descend to this, and remove the mask to co-ordinate and communicate with the passengers after the descent. My area of operations always permits a short cruise, if required, to land.
Your FMC won't tell you about avoiding the terrain on the way down,
hence the briefing when entering areas of high terrain.
That's what the crew in American Airlines #965 thought
This thread is about en-route, not approach phases of flight. Just saying that, these days, navigation in the en-route phase (while obviously it needs to be monitored) isn't "
a primary concern". I should, thinking about it, concede that it is a primary concern, but these days not a primary workload item.