> why have we decided that the FOUC (“flash of unstyled content”) ... is such a dire problem

I think the answer is that different people are sensitive to different things. I'll often come across articles going to great length to reduce some minor jank in animations or page load, and I'll be looking at it thinking yeah, I can see it, but it really doesn't bother me enough to fix. Certainly not by going to these lengths.

Meanwhile, FOUC bothers me a lot, to the point where I can't understand that its accepted in any form. I'd much rather that I don't get shown the page at all until the real font is loaded. Otherwise my eye will have tracked to the text, started reading and then there's a jarring reflow and I lose my place and have to find it again. If I'm scanning for information through lots of websites, it does mentally tire me.