A long time ago a 727 was lost at night taking off from LAX at night with two generators and another was lost overloading the final one and losing everything. That might have been the reason for standby instruments as we have now.
Back when I went to FE school on the 727, they drilled that accident into us.
But now that I think about it, this could be a different accident, but I recall they took off and essential was still on the APU, that the FE had not shut down, then just after rotation, he pulled the APU fire handle, shutting down the APU losing essential and the cockpit went dark.
Back to the accident, I've got about 500 hours in Lear 24/25/28, yes a Lear 28, and I've not a clue what could have caused this one to come basically straight down from FL280.
Sadly, there is a good chance, we'll never know.