Quote - "And frankly, so far as I can tell, you've not explained the disadvantages of broadcasting a MAYDAY on Area."
I have not read on here that anyone does not monitor 121.5 (particularly remote x country) and that the airlines all monitor 121.5 and not all airlines will be on "your Area" frequency.
So the disadvantage of a Mayday call on Area is a lower target audience number.
If a cockpit fire and only 1 short call before comms lost, would you rather a small/none audience listening or all available listening?
P.S. always set off beacon before impact and check the old batteries in the old fixed 121.5 ELT units many were just D cells and at least they have a G switch.