There are so many logical fallacies in this argument. I get the preference for having a traditional tower that you can store everything internally, but argueing that having an updated Mac Pro is going to be cheaper than an iMac Pro in the long run is way off base. For all components you’ve listed there isn’t a single one besides possibly the display that makes very much sense.
- Keyboard/Mouse - the iMac Pro’s slate gray keyboard, mouse and track-pad are already in such high demand you could easily buy an iMac Pro and resale the keyboard, mouse and trackpad at substantial profit. I’m almost certain these could easily be resold on eBay after purchase for $75 for the mouse, $100-$150 for the keyboard, and $150-$200 for the trackpad. When the iMac Pro was first shown, those are things that many got excited about.
- Display - you said yourself you’d likely get a new display eventually for the Mac Pro. Why not just keep your current display and use it with the iMac Pro. Most people that work seriously on their computers that I know of find a single display cumbersome even when it is is a nice as the 5K display in the iMac.
- Storage Components - TB3/USBC enclosure are cheap. For the money you’d get from selling the mouse/keyboard/trackpad you could easily buy a nice 4-6 bay TB3 enclosure.
Ultimately I think you are hoping for Mac Pro that is not going to be anything like what Apple will provide. Apple has already shown they are completely on board with Thunderbolt for expansion. They’ve also said that the new Mac Pro is going to be for the most demanding workloads. I see one of two scenarios for the new Mac Pro and neither one of them will fit with what your describe.
- Apple releases a new Mac Pro that looks a lot more like the trash can Mac Pro than the cheese grater one. It will continue to use TB expansion for everything except RAM, NVMe slots, and possibly a GPU. I highly doubt the new Mac Pro will have any SATA or SAS port internally, those are legacy interfaces now that would have died off 5 years ago if intel/Apple and even Dell and HP had their way.
- The new Mac Pro will likely be based on a dual or quad socket Xeon design, and the price will likely start at $7500+ without the keyboard mouse or display. That is the type of computer they mean when they say it will be designed for the most demanding workloads.
I also wouldn’t be suprised if Apple pushes really hard into the eGPU realm with an update to TB4, and support for their own eGPU enclosure, something that would benefit all their pro machines (MBP, iMac Pro, and a new Mac Pro). Just my 2 cents.