If the alternator fails you have no idea how long the battery is going to
last. As an absolute minimum you need navigation lights at night plus any instruments that are electrically powered. You will also need your GPS or the radios if you are navigating by beacons.
They did the sensible thing and landed before it got dark, sounds like there was no viable diversion airfield within reasonable range so they used the veld.
Modern aircraft with fancy electonics, glass panels, navigating by GPS etc are going to be more of a problem when the electrics act up.
Navigating at night is easy in SA with plenty of town lighting and roads with lit vehicles to follow but mozambique is a different can of worms with very few clues at night.
I would have done the same thing.