I've been doing IT for over 51 years. It is one thing to have tech expertise from both experience and training but a lot of the classes I took were in system design. You always design for the user/consumer. In the old days of Windows, the screen NEVER went black. Ever. It might "blink" but never sit there for 0.5 to 1.0 hours of a black screen. As you pointed out, there is a very diverse user group out there now. We really didn't see that until after the creation of the World Wide Web in the 90s. Before that, users were techies, for the most part. But here, we have two issues. Number one, a black screen lasting more than, I'd say, a few minutes will upset anyone who has worked with previous versions of Windows where screens didn't go black for that length of time (in most cases, the entire update used to take less than 1/2 an hour. So, you can ask around with that group and most will tell you (including the OP) that they never expect a totally black screen for that length of time and will most likely try various things to remedy what appears, to them, to be an unexpected and serious issue. (Lastly, holding down the power button or pulling the plug which are never good ideas especially with the HDD light blinking away). In the second case, with Facebook and other activities, you have people buying computers that have never seen one before. As part of my job, I had to train this middle-aged woman how to use a mouse, if you can picture that. Today, kids are practically born with a mouse in their hand and schools train students how to use a computer. But there will always be those that fall into the cracks. You said you would be "at a loss" in what people would do... well, that's not so much of an IT question as it is a psychology question. But, it is a bit after the horses got out of the barn, but I'm pretty sure those who are used to Win 3.0, 4.11, 98 SE, NT, 2000 would still be confused by a totally black screen lasting as long as it does. A screen that, with a bit of programming talent, can easily be replaced by something other than black maybe with a progress bar.
So, you think the screen MUST go black? It never did in previous versions of Windows, to my best recollection? Can you remember that happening? If you did an experiment with a room full of users doing a simulated "update" and arranged for the screen to go black for, let's say, 30 minutes, do you think some of those people would get up and comment to someone "in charge" that the update crashed or who would hold down the power button? Judging by the comments we have already seen here in Cnet on all things Windows 10 update, I'd bet many people would do those things. So, after they crash the update, they have Windows being a "mixed bag" and the user notes that something is still wrong even after restarting. Yes, I had to fix someone's laptop where they did all of these things. Ended up using my jump drive with the updated Windows on it and didn't lose any of her programs or data.
Personally, I just think MS could have done a better job. It's one thing to tell users that it will take a long time but a black screen that lasts and lasts?
As for my 12-hour update... I made up my mind that I would let it run overnight and it was the computer's FIRST update. I think the issue is that it doesn't have an SSD but, rather, a single TB HDD and is probably slow. Even with 12 GB RAM, this was a VERY inexpensive brand new computer (less than $500). So, yeah, it takes a bit of patience.