Re. 'Retina'.4K is the only widely-available, affordable option if you want a monitor with better resolution than 1080p/1440p. That's why they are "selling millions of them." Not to mention, most people don't even know that consumer monitors with 5K resolution exist.
As I already stated, 150% and 175% does not scale pixels at a 1:1 mapping. In layman's terms, everything scaled by those percentages will not look as sharp/crisp as 100% or 200% scaling. This isn't my opinion, it is fact. Since this is the LG UltraFine 5K thread, I assume people in this thread care about the picture quality of their monitor or else they would be fine with a lower resolution monitor.
Regarding "Retina":
"Retina is Apple's trademark for a display so sharp that the human eye is unable to distinguish between pixels at a typical viewing distance. As Steve Jobs said: 'It turns out there’s a magic number right around 300 pixels per inch, that when you hold something around to 10 to 12 inches away from your eyes, is the limit of the human retina to differentiate the pixels.' Given a large enough viewing distance, all displays eventually become 'retina.'"
Your eyes are typically further than 10-12"s from a computer display. Hence for these, it's more like the 218ppi of 27" 5K3K's.
But yeah, sure, I'd take a ~27" 10K, if you have any? 🙂