The issue here is that Apple's Retina resolution seems to be set as 218PPI.
Yes, both the 4K and 5K panels Apple uses in the iMacs are 218ppi. And this is why I expect the iMacs to stay at 21.5" and 27" (if the iMac Pro is staying at 27", they will not offer a larger iMac display).
Dell's 32" 8k UP3218K is 280ppi which means windows will NOT be the same size when dragged from, for example, a Pro Display XDR.
PPI has no effect on window sizing, just sharpness.
An 8K display in HIDPI mode would have an effective resolution of 3840x2160, which is the same resolution of consumer 4K displays. So windows would be of identical size, they would just look sharper on the 8K display due to the higher PPI.
You are correct in that a window on an 8K HIDPI display would resize when moved to a 6K HIDPI display (like the Pro XDR). But the XDR display is aimed at a particular market segment and likely in a single-display configuration connected to a Mac Pro or 16" MacBook Pro. As such, I would not expect anyone connecting a Pro XDR display to an 8K 32" iMac or iMac Pro just as I would expect no one to do so with a 5K 27" iMac / iMac Pro.
The iMac 21.5" is also now effectively a single-display configuration now with the loss of the LG 21.5" 4K display (I am guessing they sold very few of them as I would expect the majority of 21.5" iMacs are purchased due to space considerations so they would not have room for a second display).
The Pro Display XDR 32" is 6k and 218ppi though. If you look back at my post a little earlier in the thread a 24" monitor would be roughly 4.6K (4608x2592) and crucially stays compatible with Apple's definition of retina at the same dot pitch - 218/219ppi. And with the same dot pitch windows would remain the same size when dragged between displays.
A 32" 6K iMac and the 24" 4.5K iMac would effectively be single-display configurations as there would be no third-party display options unless Apple or LG offered them (I don't see Apple doing so, but admit LG might as they did in the past for the 4K and 5K panels though they would be many times more expensive than a consumer non-Retina display of the same effective resolution). This would probably work for most people, but if any Mac is used in a dual-display configuration, I imagine it is the 27" model since it can connect to a 27" QHD display and both would have the same effective resolution (with the 5K display being sharper thanks to it's higher PPI).
I have - and generally need - two displays for my iMac, so if they did go 32" 6K on the iMac, I'd either pay through the nose for the Apple / LG companion 6K display or hold my nose and live with the window resizing. Though to be honest, I would rather do neither. I have a very nice high-end ASUS 27" QHD gaming monitor that I use as my 5K iMac's second display as well as the main display for my Windows Gaming PC and I would hate to have to toss it.
An 8k display could not be economical, nor would it be in Apple's interests at the moment as an external one (to match the main one) would need 2 DP1.4 cables which is not elegant. Dell's
32" 8k monitor needs 2 DP1.4 cables and there's a
Thunderbolt 3 solution that does the same assuming a Titan Ridge controller - which the iMac Pro doesn't have.
An 8K display would be expensive, but Apple shipping scores of thousands a month would help knock that cost down fairly quickly. Notice how in 2014 Apple could sell a 5K display with a computer on it for the same price Dell wanted for just the 5K display - Apple's greater purchasing power knocked the price they paid down compared to what Dell was paying for much lower volumes.
As for cost, the current 8K display as Dell uses might very well be cheaper than the 6K display Apple uses in the XDR due to the extra features the XDR offers. Apple can "de-content" the XDR display I am sure, but I still expect it to be expensive enough that they could never hit the $1999 price point (and probably a $2499 one). Of course, an 8K display might very well be equally hard to hit (it retails for $4000 at Dell). Which is why I think we're going to see the 5K 27" continue.
As for connectivity, inside an iMac Apple would just create their own custom timing controllers for an 8K display like they did for the 5K display in 2014. I would not expect Apple to offer an 8K standalone display, but if LG does, it would work with a single DP1.4 lane over TB3. And the 2020 iMac Pro will certainly update the onboard TB3 controller to Titan Ridge.