Being able to replace the PSU, GPGPU, 5.25" drives and 3.5" drives is upgradable. Expansion will have to be entirely done through external PCIe chassis, and TB2 is NOT as fast as PCIe3. It's such an inelegant solution and very half-assed.
I don't just want an engineering marvel on my desk. I want it to be pragmatic and easy to expand. Using the excuse that most machines are going the other way is not viable – this is a professional machine, and every professional has different standards.
I think you hit the nail on the head. This is a professional machine, and the professionals it is aimed and, that are going to be buying this machine, are not interested in upgradability in the sense you talking about.
Find a good price/performance ratio, buy and use machine for 2-3 years, sell and buy a new one. The fact that this machine houses all the components you want to upgrade inside of it (CPU,GPU,RAM,PSU) and all the components you want to keep outside (NAS/RAID primarily), makes this prospect that much better, less waste.
Not to mention the usually excellent value retention on Apple hardware makes this a pretty darn good investment. Again, if you are in the target market that is.
Being able to replace the PSU, GPGPU, 5.25" drives and 3.5" drives is upgradable. Expansion will have to be entirely done through external PCIe chassis, and TB2 is NOT as fast as PCIe3. It's such an inelegant solution and very half-assed.
I don't just want an engineering marvel on my desk. I want it to be pragmatic and easy to expand. Using the excuse that most machines are going the other way is not viable – this is a professional machine, and every professional has different standards.