Yes, Apple could eat the costs up front with the margins they have, but that would impact the Mac's profitability for a time and that could mean it goes back to being "ignored" as the C Suite focuses on more profitable parts of the company in the interim.
This would be why I would see Apple consolidating the 21.5" and 27" into one single line below the 27" iMac Pro, and sharing the H series CPUs from the MacBook Pro - less R&D costs and shared component purchasing power. Ultimately the Mac mini would join in this with the desktop sector making up only 20% of Mac sales.
You're still going to have window-resizing issues due to the different effective resolutions. DPI does not affect window sizing, just clarity - a 27" 5K display in HiDPI 2560x1440 has a DPI of 0.1167 while a 27" QHD display at 2560x1440 has a DPI of 0.2335. The 5K HiDPI display is sharper thanks to the much finer DPI, but the windows are the same size because the effective resolution is identical. This might not be an issue for some, but it will be for others (including myself).
I might have missed something but Retina displays have all been the same PPI (hence my calculation on a 24" Retina display being 4608x2592 - 4.6k) and my understanding was that, for example, a window moving from a 27" iMac 5k with a secondary 21.5" Ultrafine LG monitor at the same effective PPI would appear seamless even though the 21.5" may would represent a smaller viewport. There may be a jarring of window sizes if moving to a secondary 27" 1080p monitor became the effective PPI would be different.
For what it's worth, I seldom use a secondary screen like that so it doesn't bother me.
I am pretty sure there are no technical limitations with using a T2 chip with a Fusion Drive. With Startup Security, you can boot a T2 Mac from an external HDD so there should be no reason you cannot do so from an internal HDD. And Apple could easily ship a T2 iMac with a version of macOS that requires it (or perhaps just certain parts) be located on the SSD part of a Fusion drive to work with "Full Security" mode.
I'm guessing that Apple started with SSD-only configurations for T-series chips because:
a) it was easier (since APFS is designed for SSDs)
b) they could exercise full control either through the storage modules being soldered to the system board or using only Apple-sourced blades.
That's a decent point but it's somewhat moot as people have been saying for some time now that Apple Macs should be all-SSD given the price that they are.
When even the Mac mini went with T2 and all SSD (and a massive price increase) with a more powerful desktop CPU instead of sticking with mobile CPU and keeping hard drives in as an option the writing had to be on the wall for the iMac's hard drives.
It's already a no brainer with laptops because of space considerations but I think hard drives are on the way out from desktops anyway so it's going to be inevitable that Apple have to go all SSD at some point.
Hard disks take up valuable space in designs. Without a hard drives the iMac can can be redesigned and while the reclaimed space in the iMac Pro was taken up by the cooling solution there's an obvious opportunity to create a really thin iMac without hard drives in.