Sure they had to start with that and build that up, but now imagine they come out with a TRUE desktop SoC that has plenty of PCI lanes, allows eternal RAM, ECC, etc... What would that mean?
Well, it would mean a very expensive system to pay for the development of a "desktop" SoC that was only ever sold in relatively tiny quantities in Mac Pros and hypothetical "pro" iMacs.
It would mean a system constrained by the same bottlenecks of external RAM speed, separate RAM and VRAM, PCIe speed and the capabilities of 3rd party graphics cards as competing Intel and AMD systems. (...and if you're going to have external RAM and GPUs how is it still a "system on a chip"?)
Supporting discrete (non-Apple) GPUs would remove any incentive for the makers of high-end software (the sort of specialised stuff that sells to potential Mac Pro customers) to optimise it for Metal/Mx Ultra.
It would probably give Intel and AMD a good run for their money - but it would be very hard to create an Xeon/Threadripper-killer: going for external RAM, PCIe graphics etc. would throw away the advantages of fast, unified RAM and GPUs optimised for Metal and the economies of a "true" SoC (no PCIe, RAM, M.2 slots etc.) that have helped Apple thrash Intel in the thin laptop/small-form-factor market.
Apple Silicon/ARMs biggest "killer feature" is still computing-power-per-Watt. That's a killer feature in laptops. It's important in SFF (if you want small and quiet). It's also important in server farms/"high density computing"/cloud applications - hence the likes of Amazon's Graviton - but Apple don't have a horse in that race. But on a "big box'o'slots" high-end personal workstation like the Mac Pro, alongside a quartet of power-guzzling discrete GPUs, it may be "nice to have" but it's hardly a killer feature.
Economies of scale are everything with electronics - and Apple have a very healthy slice of the global market for the sort of prosumer laptops and small-form-factor systems that can used what is essentially re-purposed iPhone tech (which Apple sells in even larger quantities). What they've achieved so far is to use the same building blocks to cater for everything up to - well, whatever people are already happily doing with M1 Max & Ultra. The chip you are describing would be
just for the replacement for the 2019 Mac Pro which is
not a big seller and only significant in the "people who are wedded to Mac-only software" market.
I'm not saying that Apple won't produce a workstation-class Apple Silicon chip, but to me it sounds like a vanity project, and I'm half expecting the "new Mac Pro" to be, essentially, a rackmount version of the Mac Studio (with Mx Extreme processor if they can join 4 Maxes together without mounting two of them in hyperspace).
In any case, the desktop market is being eaten away by more powerful laptops at one end, faster external peripherals via TB4/USB4 in the middle and on-demand cloud computing at the high end. It's not going away any time soon, but it's hardly a growth area to invest in.
And why do we STILl not have a 27/30" iMac/pro?
...because Apple are once more selling a choice of external displays (possibly with more in the pipeline) and a much improved range of headless desktops. A Mac Studio Max/Studio Display combo costs about the same as a top-end i9 iMac with 32GB RAM - and has comparable performance - while a Studio Ultra/Display combo is a
lot cheaper than a comparable 18 core iMac Pro was. For the sake of two extra cables you get the choice of two Apple displays (SD or XDR), a plethora of 3rd party options and the opportunity of replacing/upgrading the computer and display independently.
Plus, a lot of people who previously had a MBP for the road and an iMac on their desk for power are now going to realise that a Mx Max doesn't suffer much from being in a laptop and they don't need a separate desktop any more. Hence the Studio Display, which actually makes more sense as a MacBook docking station than as a display for a desktop Mac.
Maybe we'll see a new 'large' iMac if/when there's something better than a warmed-over version of the 2017 5k panel to put in it. I mean, its great that the 2017 panel is still so good 5 years later, but I don't see it staying top of the heap much longer and wouldn't want to have one welded into a new computer today.