I think you need to look at Apple's support documentation for 4K monitors and sorry to say but most 4,1s and 5,1s on this forum have been upgraded enough to make 6,1 look long in the tooth and they can continue to be upgraded to next gen graphics, next gen video ports, USB 3.1, NVME solid state drives, and Bluetooth 4.2.
So my opinion that a sealed off box is hobbled and lame cannot be any truer. That doesn't mean your feelings should be hurt because of the investment you made - after all a computer is as useful and productive as its owner no matter what it's shortcomings are. If you can do great work with it then all power to you.
Feelings hurt - on an internet discussion forum? You gotta be kidding me.
I had the same reservations as you and waited many months before getting the nMP. It actually has less spaghetti hanging out the back of it than the old one. It's been upgraded to the hilt: flashed to 5.1, the hottest CPU you can put in it (W3690), more, faster RAM, USB 3, 700mw wireless AC (picks up Mars and is fast), two SSD's plus a RAID, GTX 670 SC with a 970 OC on the way. Couldn't care less about new BT...don't use it on the cMP. Like I said, it is serviceable and has its strengths. The problem is, there are bottlenecks with the old architecture. The biggest is that I'm often enough CPU limited and there is nothing that can be done about it...period, and that's not the only limitation to further upgrading the 5.1. Think you'll ever see PCIE 3 or faster RAM?
No, I too wanted a modernized cheese grater. We didn't get it. What we did get was something that has its own unique advantages that, when utilized for its intent, is remarkably capable and quiet. Upgradeability is an issue for sure. It wasn't all that easy on the cMP except for adding drives. For the nMP CPU and SSD upgrades are already out. We don't know for certain GPU ugrades won't be available at some point. The possibility of an additional external GPU (Nvidia) is near for those apps that would benefit (think CUDA). I/O is not an issue with DP 1.2 and HDMI 1.4 and TB interfaces. 60 FPS ingestion isn't insurmountable, but is as rare as mule mammaries. 30/24 FPS is the standard for in/out in 4K/UHD and that's not a problem, nor is SST display at 60 Hz via TB/DP.
As stated, I have and use both Mac desktops, each for certain purposes, and while not yet perfected, the workflow is, well, working. For the bleeding edge in CPU/GPU mobo, etc., I'm soon building a PC for Win apps that will absolutely trash two of anything Apple makes, and thats coming from a guy that has one or two of everything Apple makes.
As for the nMP, don't knock it until you actually tried it, preferably in a professional setting. It works...very, very, well. As someone who actually owns both, and uses them to make a living, I don't have any regrets, but admit I can't wait until after the first of the year to build that PC with an overlocked 4790 or 5690, GTX 980, and all SSD storage in RAID with no archaic architectural bottlenecks. I can even ignore progress and put a 5 1/4 floppy in it if I want
PS - Forget the Apple documentation you cite. It often does not reflect reality. That's what they have tested and will support. The 4K page is WOEFULLY outdated and higher level support and engineering knows it. Show me where they say you can flash a 4.1 to 5.1 and stick an Nvidia 5XX or higher in it.