"For the average user" -- and this is my problem with nearly every objection to the Mac Pro.
Everyone complaining about the Mac Pro, or any of these other topics related to it (eg. SATA vs NVMe) in some form or another - including you - keep making their claims based on that statement: "for the average user".
This machine and all the various options for it are
not for the average user so every "for the average user" statement is pointless and meaningless in a conversation about adding expensive storage to a genuinely professional machine.
Needless to say, I'd even argue that a lot of people who use Macs in general (not just Mac Pros etc) are not "the average user".
At least some of "the average users" appreciate even the simple things like faster startup times, snappier web browsing (cache stored locally, etc), snappier, well, everything, because that's the end result of faster storage. You don't have to be editing large databases or videos to notice a difference. I know, because my parents and sister notice the difference and they do "average" tasks like email, web browsing, accounting, writing, basic photo management and editing.
There's another example - the average user will notice the difference between SATA and NVMe scrolling through a few thousand photos in the Photos app. Again, my family members did when they made that change.
As for disk space requirements are too much for editing uncompressed 4K. Well ok, maybe so. Or maybe I am installing 40TB of striped 12GB/s+ NVMe drive arrays in my PCIe slots if I want to (no, personally I'm not, but I imagine Kevin Feige wouldn't have any trouble doing that if he wanted to). Sure, that's probably not the best solution. I know for what I do - large databases. If I was in the market for something like this and I was going to be spending any significant time working on them, my workflow would be that I'll copy them off the SAN onto my internal 12GB/s NVMe array, work with it, then copy it back. Depending on what I'm doing with it.
The point is, as always, this machine provides options. And if I sound like I'm on a rant, it's because so many people here keep pushing their opinions based on their own workflows or some narrow limited imagined workflows, and just can't see past those to see how this thing provides enormous flexibility and options for all kinds of workflows. Which is the whole point of it.
So to your next point about how overpriced you think it is. Compared to what? As others have done here and in multiple other places, spec it out against comparable Dell servers, or HP workstations, or any number of other competing machines and it's not overpriced at all. Ok, maybe you want to compare it to the high end iMac or iMac Pro. Sure, spec for spec (CPU, GPU, storage, RAM) yep, it's more expensive. Why do you think that is? Because those machines don't have the EIGHT PCIe slots, the insane cooling, the modularity, the engineering that makes it all work together properly, and all the other things that make this what it is and what those machines aren't. If you want the CPU, GPU, RAM, and storage, all at iMac prices, then get an iMac. If you want all the other stuff that this machine has that makes it more than those then you don't get that for free.
As for your "Threadripper" example. Spec one of those out spec for spec and tell me how this is more expensive. And no, "build-your-own" doesn't count. The other thing that this machine comes with that its target market needs is that they buy it, pull it out of the box, turn it on, and get to work. For these people their time is their most valuable commodity. They get paid multiple hundreds of dollars per hour for the professional WORK they do. Every hour they spend building their own machine - or even researching all the ways they can buy third party RAM instead of Apple's RAM and install it themselves - costs them significantly more than what they'll pay Apple to build this for them instead. If you don't understand that then you just don't understand who this machine is for. And if you're not one of those people whose time is worth something significant then this machine is not for you, nor is it for anyone else on here making similar arguments.
One more thing: Your Threadripper machine doesn't have macOS. macOS isn't free. There's a reason Apple's license agreements say you're not allowed to run macOS on anything other than Apple hardware (even virtualized). That's because Apple's hardware is what pays for macOS. How about Apple change their business model? What's macOS worth? I mean it must be worth something, otherwise why are we even here arguing about this? The ONLY reason anyone would want a Mac Pro instead of a similar specced PC, if they genuinely think the Mac Pro is expensive, is for macOS, right? The only reason we're here complaining about or even discussing Apple stuff is not for the hardware, it's for the OS. Without macOS, there are an enormous amount of alternatives to choose from and Apple becomes a very niche hardware maker with no significant differentiator. So therefore macOS is worth something, for Macs in general to create all this hoopla, right? So what's it worth? Should they just give it away for free? Hardly. You're not going to agree with this but I'm going to argue that it's worth a LOT more than Windows 10 - why? Because it's worth all this fuss. If it's worth all this fuss then it's better than Windows 10, at least for the people who are making the fuss. And it's better because it's had a lot more time, R&D, etc. put into it than Windows 10 has. And even if we look at the $200 price of Windows 10 alone, that's before you include the virus protection and other stuff you have to buy to go with Windows to make it anything close to on a par with macOS. Whatever we price it at, it's included in the price of every mac. So factor that in when you're speccing out your Threadripper machine as well.
I just want to see someone throwing around all these arguments about how this Mac Pro is overpriced show me something
comparable by any other decent manufacturer, that's significantly cheaper. No one's been able to do this yet. Anyone want to step up to the plate on this one?
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Yet Apple keeps selling millions and millions and millions of them... so weird.

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Why? What's wrong with adding a PCIe SSD card for storage? Why is this such an issue?
You effectively do have T2 as an option (for storage) because you CAN add SSDs without Apple service. That's one of a million options the PCIe slots are for! Are you saying they got that wrong and you should be able to add storage through some means other than PCIe?
If you don't like the internal drive and its reliance on the T2 then ignore it. Add whatever cheap or expensive storage you want in the industry standard PCIe slot and boot off that. There... full control to you.
What on earth is the problem???