I expect the 27" model is more popular than the 21.5" and by a large margin. I also expect it would be more popular compared to a ~24". And if the rumors of this ~24" model being targeted as a "less expensive" entry, it might not even be a Retina display (by, for example, using the LG 23.7" Ultrafine 4K panel, which is not Retina). In such a case, I can't see it being very popular at all compared to the Retina display of the 27" 5K.
I've mooted before that the 21.5" panel is increasingly popular because of the size of the panel rather than the resolution on offer. I don't have anything against the panel size per se because of the resolution (4096x2304) is actually offers more pixels than in a less dense 3840x2160 in the 23".
But it will still be a $4000+ computer. Sure, you can spec an iMac 5K to that if you choose most every top-end BTO option, but the iMac 5K starts at $1800 and even what many would consider a "minimum acceptable configuration" (i5, 16GB, 512GB SSD, 575X GPU) is $2499.
Desktop Macs are already almost not worth Apple's effort with their 20% overall share and the iMac - and iMac 5K, specifically - are the bulk of that 20%. Limiting their options to only "spend (a lot) more if you don't want to settle for less" does not strike me as sound marketing.
I am confident the rumored ~24" iMac is meant to replace the 21.5" model. I am also confident Apple will continue to offer the iMac 5K with a 27" display.
What I am not confident about is the pricing and positioning of the rumored ~24" model. If they want to make it a way to get into the iMac line for less money, I think it might go non-Retina (by using the LG 23/7" Ultrafine panel) and will definitely start with a 1TB Fusion Drive and maybe even a 1TB HDD.
If they are comfortable charging as much (or more) than the current 21.5", then I think it could offer a new Retina panel and perhaps this panel would also be offered by Apple as the new Thunderbolt Display to be used with the Mac mini, Mac Pro and MacBook family (with the lower-quality LG UltraFine 23.7" sticking around as a lower-price option).
Regarding the iMac Pro. A repriced current SKU would see potentially hundreds of dollars saved off the price of the Intel CPU, the RAM, the GPU, and NAND flash - all of which was agreed at 2017 prices and have remained that way ever since.
If we assume that Apple would not be dropping prices of the iMac Pro as per their usual policy, they could in theory refresh the iMac Pro with W-2255 (a 10 core, 20 thread CPU), VEGA56, 64Gb RAM, and 2Tb SSD with no change in price.
That's one heck of a model but any new iMac Pro lower SKU could dispense with the VEGA56 (which itself could be replaced by an AMD 5300-5600 Pro for example) and SKUs with AMD 5300 Pro for a further reduction in price.
The way I would have brought a cheaper iMac Pro SKU in would have been to offer it with a smaller panel - a 4.6K 23.7" for example - it's not clear if a 4k non retina panel could be 'Pro' enough. Maybe going all-SSD and offering reasonably high spec and 4 Thunderbolt ports is enough?
In terms of consolidation in a shrinking market, Apple could easily make savings on an engineering side of things by reducing their parts bin. They are already letting LG shoulder the burden of offering the 23.7" 4k display (with Thunderbolt) in their Apple Store.
This is where something 'drastic' like killing off both the 21.5" and 27" in favour of a single non pro SKU would be one way of consolidating their efforts in the desktop sphere. As you say, the engineering effort for desktops may no longer be worth it for Apple across the many distinct SKUs which is why I have been suggesting they base future efforts around something like the Comet Lake H CPUs that will go into the MacBook Pro 16".
If Apple have the chance to add the T2 to a slimmer iMac I can't see why they would want to continue to cater for spinning disks in a new design.
A 23.7" iMac Air is their chance to dispense with hard drives in my view and given the pricing of the monitor panel in the Apple Store is broadly the same we could assume that the cost calculation could easily be based off the existing retail prices of the 21.5" iMac.
Looking at the 21.5" iMac and seeing the advances in NAND pricing that Apple have had I think they could establish a starting price of a 23.7" iMac with T2, 8Gb RAM, and 256Gb SSD to cost from $1299-1499 - if they thought that 256Gb was an acceptable base model. For me, there's just no getting past the idea that a new size format iMac is the big chance for Apple to ditch the hard drive while letting the 'old' iMacs linger with the slightest of updates.