You say: "Let's hope Apple gets on the stick with new ID for the iMac and iMac Pro. The Pro is an important bridge between the iMac and the Mac Pro and the iMac 21.5" and 27" are important to Apple, but really long in the tooth."
I agree, but I wanted to make a comment re: the 21.5 inch iMac. I think a lot of people have kind of overlooked the LG 4K---the new one. It's 23.5/24 inches. I think the iMac is going to get a redesign and Apple will release a 24 incher at 4K and possibly kill the 21 incher.
I mean, the LG 4 and 5K's were always the same size as the respective iMac's; why make a 24 inch display if a 24 inch iMac is not right behind?
Good reasoning there.
The LG Ultrafines are 16:9 23.7" 3840x2160 which is lesser resolution than the 21.5" offers (4096x2304). Apple could straight up offer a 24" iMac at that resolution.
They could also opt to continue the 'full' 4k screen resolution from the 21.5" in a 25" package at the slightly higher resolution too (using panels with the same dot pitch as the UltraFine) - and rather than killing the 21.5" model they could leave a 21.5" model behind while a 24/25" model goes all-SSD.
If they want to remove the old retina 21.5" panel from the supply chain they could in fact keep supplying that horrible 2017 model with the 2.3GHz i5 mobile CPU.
The one issue I see with the new LG 4K Ultrafine is it is not a "true" Retina display at 189dpi. At "typical" viewing distances, Apple defines a Retina display for a Mac desktop at ~220 dpi and the 21.5" has a DPI of 219 and the 27" 5K's has a DPI of 218.
Mind you, 189dpi is better than just about any other 4K display on the market by a decent margin and almost as good as the Microsoft Surface Studio's 193dpi, but it is still in "bad zone" if used in HiDPI mode. And the LG 31.5" 4K panel has a DPI of only 138, which is also in the "bad zone". So I am skeptical Apple would be willing to use the 24" panel in a new iMac (and the 31.5" is a definite no-go).
Apple's 32" ProXDR is 6k - no chance of an iMac Pro coming with that panel I fear. 24/25" would give the physical size that most business users are used to as 21.5" is something of a budget size screen even though.
The issue that @Glockworkorange has mentioned is that Apple no longer sell the 21.5" 4k LG monitor any more. So buyers can't purchase a 'matching' external screen.
A 24" or 25" 4k screen may be seen as better value by Mac buyers or as foreshadowing for a forthcoming product.
Or Apple could make their entire 21.5" line retina with a form factor change to go along with new CPUs and GPUs - to include going all SSD - and hope that interest is piqued by that change. But would it interest many people who want a bigger screen without going 27"? A fully SSD 27" is surely going to increase in price too.
Could a new generation 21.5" 4k iMac be the one that goes with the H series mobile CPUs to add more cores - becoming a desk bound 16" Macbook Pro with bigger 4k screen and perhaps a VESA attachment option?
I'm still of the opinion that Apple sells an expanded iMac Pro range of SKUs at 27" - maybe with a redesign - and a 24" 4k becomes the workhorse of the range. Apple will probably then cheese off a load of fans by continuing to offer a poverty spec 21.5".
When intel (for CPU) and/or AMD (for GPU) put out something that is actually an upgrade.
If they were to "refresh" it right now, they would be pretty much identical parts with different part numbers only.
The new CPUs have a significant price cut thanks to AMD Ryzen competition. Apple could in fact secure a price cut to keep supplying the old CPUs but would Intel do them that deal? Apple may be happy on the one hand to take the old CPUs because of the heat profiles in the existing enclosure but letting hardware stagnate without meaningful refreshes is what was threatening a sales collapse on the 2013 Mac Pro by professionals.
Keeping the 2017 iMac Pro on the books without a refresh (even if it's to double the RAM, double the storage, or even bump the base CPU to the 10 core model for example) would be very bad news with pressure from below coming naturally in due course due to Comet Lake S and potential 10 core/20 thread CPUs.
Like the Mac Pro - the reason to update the iMac Pro would be to show professional buyers on a budget that Apple still mean business.