I have a 40in 5k (32in 4k, but wider). IMHO, 138ppi is the bare minimum, but it really depends on a person's eyesight and preferences.
I would love a large-ish ultra-wide with > 160ppi. One day, maybe, that being said, by that time those things will exist and be reasonably priced, my eyes might not be able to appreciate the difference.
I'm using three 4k 32" screens arranged vertically, for 6480 x 3840 desktop size.
The only real monitor upgrade I'm willing to entertain is a ~50" 8k curved screen (basically a curved TV-sized screen), which has not been made yet AFAIK. I'm not into "ultrawide", for me it has to be "ultrawide" and "ultratall". I want all that screen real estate in high PPI.
I tried test-driving a 50" 4k TV for a week and the flatness of it was not what I wanted, it has to be a curved screen for workstation use.
100% I used to have a 43-inch 4K "monitor" (16:9). The lack of curvature meant that at the sides of the screen, there was noticeable color shifting due to variations in viewing angle. That's with an IPS panel.
Your dream is probably a ~50in 8K TV (with RGB pattern if you are on macOS), but curved. I don't know if that will ever exist.
Personally, I found that with a bigger 16:9, I would not use the top and bottom of the screen. When I "downgraded" to a 40in ultra-wide, there was not much difference in the space I was using.
What planet are those people on? That's Gucci bag territory. They can take their res and shove it, that's almost NINE GRAND (granted, Canadian pesos) for a freakin display! Who is this for, just Pixar employees?
And how much do you think the Humanscale mount costs that you would otherwise use? The Pro Display XDR is too heavy for a $30 Amazon Basics mounting arm or anything similarly cheap.
The Pro Display XDR is 7.48 kg and a $199 Ergotron LX Pro is rated up to 10 kg, so that's a fifth of the Apple stand's price.
And looking at Amazon.de listings, it's definitely possible to buy a cheap arm that's good for 7.5 kg. A "suptek Monitor Mount" is good for 10 kg according to the listing: https://www.amazon.de/dp/B0833NQ8CR
In terms of pixel count it's between Apple's 5k and 6k monitors, and its pricing is between the two. It's also far lower pixel density. So, not really.
Some other specs: refresh rate, 120Hz; brightness, 400 cd/m².