Thanks for all the replies re FMS VNAV.
So, as I understand it, by any of four methods, a live human could climb from 35000ft to 39000ft and level off at 39000ft and, once that had all been achieved:
(1) - Input to FMS any number of existing (or their own) waypoints
(2) - Input to FMS a 5000ft level at a chosen waypoint using a chosen descent method, such as, for example, idle descent
(3) - Enable 5000ft by dialling 5000 in the (MCP?) window, with a press to pre-arm.
After completing (1) to (3), which might require nothing more than 10 minutes after levelling at 39000ft, said live human could then die (by depressurising, or, if already depressurised, by removing their oxygen mask) and the aeroplane would spend the next several hours automatically flying over the waypoints in sequence, descending when it calculated necessary to achieve 5000ft by the location programmed several hours earlier.
Are there any flaws in the above, please?