Many parts might, technically, be user replaceable, but the firmware needed to make it work is not necessarily available and the component cost is currently crazy.
I am looking to get a Mac Pro later this year but at this point I am happy to get a reworked "old style" 12 core 3.?? for under $4,000. Simple user upgrades, no firmware issues and not cost prohibitive. You can upgrade them to at least seven internals, 128GB RAM, etc etc, I don't need to pay through the nose for a graphics card that can run six screens effortlessly, or four 4K screens, I can't afford one 4K screen yet! I am sure they have a market for high end video and graphics pros, animation studios and special effects houses will buy them by the dozen, but that isn't what I need to work 4-5GB .PSB files and even when we inevitably go to 30+ MP, I still won't.
For a photographer the only thing Thunderbolt does better than SATA is lighten their wallet.