Ah, I think this is becoming clearer (no pun intended). The computer my MBP replaced (to which I have the 4K monitors attached) was a late 2013 27" iMac. It’s native resolution is 2560x1440. That’s the same as what I’m currently using on my LG 4K monitors, so they look great to me.
So it’s likely I’ve never seen a 5K screen (at least not a 27" one). If I did, I might say “What was I thinking? This is so much better!”. But for now, ignorance is bliss. 
After getting burned badly when I bought the original 17-inch 2001-2004 LCD/ACD Apple Studio Display which lasted only about three years, I find it hard to justify the high price of Apple displays.
However, 20 years after that fiasco, what determines my display choice now is avoiding eyestrain (i.e. headaches), which I do by using a Dell 27-inch 4K display set at the default 1920 x 1080. My aging eyes are much happier with large type on a large screen than getting the full resolution possible.
That “1440p” resolution on a 27" panel (without scaling) produces the same 110 ppi that Apple optimizes their UI for.
Running a 4K screen at an effective 1440p resolution produces the same size, but with non-integer GPU scaling (hence GPU load and fuzziness issues reported by some).
… And that means you’re using 2x retina scaling, so there’s a sharp image with minimal GPU overhead. And if your eyes are comfortable with the larger objects (due to an effective 82 ppi), then it’s a win all around.
Does anyone remember when Steve Jobs said, on stage at some keynote, that Apple thinks the optimum ppi is 100? I think he was introducing a Powerbook with a 100 ppi resolution. It would have been after OS X came out.
This is why it bugs me that at some point they started releasing computers with displays at a higher PPI than 100. It makes everything look smaller than it should be. And, it is no longer WYSIWYG (What You See Is What You Get), because if you compose a document and then print it, the text on the printed copy is larger than what you saw on the display.
For example: the display on my 2013 MacBook Pro is 3456x2234 at 254 ppi. At the default 2x Rentina, it is still 127 ppi, which is a lot smaller than 100. It is even nowhere near 110.
Sure, I can get a more reasonable resolution through fractional scaling. But that’s not the point. They’re producing devices which resolutions that are non-optimal for macOS.
Yes, but remember another Jobs quote: “Real artists ship”. Idealism needs to make way for what people want to buy.
A 15" laptop screen at 100 ppi would be 1311x737 pixels - slightly larger than 720p. Even at 110 ppi, that would still only be 1443x811.
And looking at actual products, my 11" MacBook Air’s screen is 1366x768 (135 ppi).
How many people would want to use such a laptop? Most applications are optimized for more pixels than that and would not behave well on such a screen. My 11" Air is a perfect example - I need to disable all kinds of toolbars on my apps in order to have enough usable space for document content. And quite a lot of web pages end up covering half the screen with advertisements, menus and toolbars. It’s not a pleasant experience.
A comfortable 1080p screen at 110 ppi would be a 20" screen - larger than anybody would ever want in a laptop.
Again, idealism needs to take a back seat to what people will actually buy. And if you think it makes UI elements too small for comfort, that’s why display scaling exists.
Jonathan, I replaced the Thunderbolt cable between my Mac M2 mini and Studio display about 3 days ago and so far I have not seen any of the previous blackouts or system problems. The new cable is from OWC. Thank you for your suggestion!
Reading the links to the Amazon reviews in this article convinced me to get the base Apple Studio Display, which Amazon is currently discounting by $300:
Asus and Benq have new 27" 5K monitors out now/soon. Attractive for me as I need built-in ability to pivot into portrait. Matte display would be great too.
I’m in the UK so I’m not sure when they’ll reach these shores. Edit: Benq will be Feb 2025 in Europe. Asus still TBC but they are promoting it already.
Asus ProArt Display 5K PA27JCV
Benq PD2730S｜27" 5K 98% P3 Thunderbolt 4
I bought a Studio display (base model) earlier this year. The first was defective and had to be returned but I’m extremely happy with the replacement. FWIW.