$200 for a PSU is enough to get an amazing 1kW power supply or a good 1.3kW. Neither of which will be even 50% utilized by a base model Mac Pro spec. Under full load, you would be using less than 500W with just a 580 and a W-3223. But sure, add another $50-100 and get a 2kW PSU if you want. You're still at around 1/3rd of the Mac Pro base price. $50 is enough to buy an aluminum E-ATX case. Even a poorly designed one has no issues with thermal flow. And it includes fans. Any cables required are included with the motherboard. The factory heatsink/fan combo is perfectly fine, and there's no need to replace it. And I hate to break it to you, but EVERY enclosure is "custom designed". Just because Apple built one around one specific motherboard whereas every other case is built around a standard, doesn't mean that there wasn't a design team involved.
The motherboard specs a 205W CPU because that's the biggest one Intel makes. Every component here has a warranty. You don't have the custom controllers, granted. Go ahead and add $50 to the price for a Thunderbolt card. You can still build 3 workstations for the cost of one Mac Pro.
If your workshop requires MacOS and FCPx, you buy a Mac Pro. Nobody's refuting that statement. You pay the extra $4000 because you're locked into an ecosystem. It's the only legal way to run the software. However, if you only care about the output and not the workflow, the value isn't there. When the next-gen Z-series or Precision workstation comes out in a few months for $3000 with the same specs as the Mac Pro, which would you buy? Honestly? As a business owner?
With the warranty, you're expecting a business to RMA out PSUs and video cards, instead of having an inclusive warranty over the whole machine?

. There's nothing to win here with your argument, you believe that entry level components behave as well as custom (yes,
custom) designed components. I don't, and I don't believe that building your own machines at a business where you depend on the machine for your livelihood is good practice.
As a business owner, having quite a few "iBuyPower" rigs from best buy in the office, I get your argument. You can get a lot of computing bang for your buck with Windows, and we have workflows that run on Windows (CAD/ Architecture). But boy are they gaudy, and we've had our fair share of issues with them over the years. When the new Precision stations come out, I'll have to take a hard look. There is a current machine (
not sure if I can share this config link correctly) that costs $3500, and does not have as much expansion capacity, and is older architecture. Even with a lower cost, the lion's share of our IT budget goes to supporting Windows machines, where Macs drive substantially less tickets. The total cost of ownership on Mac is lower for us, even with the higher starting price.
My big issue with your comment is again, you are misrepresenting value to prove a point. You're in the ballpark on the component cost, generally, but like others have done with the iPhone, is it really just the cost of the components that determines worth? The seamless engineering, design, warranty, and software integration have no value?
Like most forums, there's no right or wrong answer, but just parading that you can get The Same Thing™️ off of Newegg is factually incorrect. People who buy these don't just have wads of cash rolling around and are ignorant of specs, they're thoughtful and strategic with their purchases and make sure they align with all of their values (not just how cheap it is). If
you don't find value in this machine, then just don't buy one. If my business required this level of processing to be competitive, I would be the first to preorder it.