"Still, there are two SSD storage slots for graphics, media, and networking cards"
Can I have some mayo on that word salad, please? Presumably means PCIe slots.
...but the 2019 MP has 8 slots, which take up 3/4 of the internal space. If the new MP only has 2 but stays the same size then it's going to be the most expensive box of fresh air, ever.
My big note of caution here is this: imagine Apple is testing out prototypes new Apple Silicon SoCs (M2 Pro, Max Ultra, Extreme, Ludicrous, M3, whatever) and wants to get them in the hands of software developers then the obvious thing to do is to make a large, spread-out development board with plenty of space for patches and connection points for test gear, with a PCIe slot or two for networking cards etc. (even the M1 seems to have
some PCIe lanes for ethernet etc.) ...and if it even needs a case, stick it in a surplus Mac Pro case with more power and cooling than you need to worry about. It would be no big surprise if systems like this are floating around, and if some leaker sees them they'll think "scoop! new Mac Pro!" but in reality it's just a breadboard job that is never going to see the light of day.
As for these bombshells:
Non-upgradeable RAM:
That's inevitable if it's going to use anything resembling the current Apple Silicon SoCs. M2 Max/Ultra might increase the maximum by supporting larger LPDDR5 modules (or larger modules may have become available) although the major RAM bandwidth upgrade already happened with M1 => M1 Pro/Max. Nothing to see here.
Non-upgradeable storage:
If there are PCIe slots, you can add internal storage (dedicated SSDs for PCIe slots, or PCIe-to M.2 cards). The 2019 Mac Pro even has mounting points & connectors for a 3rd-party spinning rust module.
As for the primary, proprietary Apple fast SSD storage - the 2019 Mac Pro launched with these as "non-user-upgradeable" with the same justification as for the Studio*: they're raw flash storage and just changing them won't work because the controller on the T2 (for the 2019MP) or M1 (for the Studio) would need to be re-configured. It's possible - but seems pretty unlikely - that there are actually
physical differences between the controllers in different storage configs, but more likely it is is just down to whether Apple chooses to make the necessary function available in their configurator tool. In the case of the 2019 MP, Apple later offered user upgrade kits - no movement so far on the Studio.
* Some are claiming success upgrading the Studio if you install modules in the correct permutations but Apple won't sell you the modules and I don't fancy buying and dismantling
two Mac Studios to test it out!
GPU support:
Whether or not you can plug in 3rd party (well, AMD, at least) GPUs is really the big story here - so far, Apple have only been supporting their own GPUs on Apple Silicon. If it definitely supports PCIe GPUs that
would be a scoop, rather than something to mention in an aside. They'd also need a
lot of PCIe lanes (GPUs are one of the few things that actually make use of x16 slots).
PCIe support:
Plenty of uses for a couple of PCIe slots other than GPUs (esp. if you don't want a rats nest of USB-C cables and dongles hanging off the back) and some users could fill a lot more than 2 slots with audio/video interface cards. However, many of those cards - unlike high-end GPUs - would be fine in external 4-lane TB-to-PCIe enclosures which could be rack mounted along with a Studio-like computer to reduce the clutter. (One of the faults of the Trashcan was that it was exactly the wrong shape for rack mounting - there are already units available that can mount a Studio alongside 3xPCIe slots). Of course, rackmount kit tends to be "serious callers only" prices - but no more so than the 2019 Mac Pro itself.