It's disturbing to see an OS update seemingly do something with the BIOS to render the machine completely inoperable. I wonder if the EFI firmware on Macs is just as fragile as some PCs, and something like this happened: https://news.ycombinator.com/item?id=11008449
If that was what happened, and you were living in a place like Shenzhen, there's a chance you could simply go to one of the many repair shops around and have the BIOS reflashed in a few minutes while you wait, for <$10USD, and be back to working again.
Of course, Apple is heavily anti-right-to-repair and this might not even be an option in the future... and naturally, the reasons they give are "for security" -- securing their profits, that is.
The other is the overall "opaqueness" of Apple products, which reminds me of this classic Douglas Adams quote: "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair."