There isn't a lot of differentiation between the Mac Studio and Mac Pro as of right now, so Apple may be planning for a more powerful M4 Ultra variant for the Mac Pro.
The Mac Pro has internal PCIe slots which provide a
lot more I/O bandwidth than you can get with external Thunderbolt-to-PCIe enclosures. If you need lots of fast internal storage, or multiple specialist I/O, networking or AV cards
that can take full advantage of the bandwidth of 8x or 16x slots you need this (and will presumably be prepared to pay through the nose for it). That's the only distinction that counts, and the only reason for the Mac Pro's continued existence. As I understand it, most of that PCIe bandwidth came from the surplus SSD interface on the M2 Ultra's second die.
If the - quite considerable - Thunderbolt 3/4 I/O of the Mac Studio Ultra is enough for you (and that's going to improve when TB5 rolls out), then there's no point in getting a Mac Pro-like "big box o'slots" just to get extra CPU and GPU cores and RAM capacity.
Part of the disappointment with the 2023 Mac Pro was down to the name (and cosmetic design), which invited comparison with the 2019 Mac Pro. I think it helps to mentally re-name things:
Mac Studio -> New Mac Pro (the trashcan, done mostly right)
2023 Mac Pro -> Mac Pro PCIe (for those who need lots of PCIe)
If you want something like the 2019 Mac Pro then Apple Silicon isn't the tool for the job - adding external, expandable RAM and 3rd party discrete GPU support undoes some of the key advantages of Apple Silicon. Using conventional third-party GPUs would mean that the Mac could never be better (or offer something significantly different) than AMD's (or NVIDIA's if hell freezes over) latest chipset.
If Apple thinks there is a niche for a specialist AI server system (as has been rumored) and wants to go head-to-head with Nvidia's Grace/Hopper, that's a whole new ballgame. I don't think the 2019 Mac Pro-style "general purpose personal workstation" has a rosy enough future to warrant developing a whole new die for: the market is being eaten by ever-more-powerful laptops and small-form-factor systems at one end, and pay-for-what-you-use cloud computing at the other.
The Mac Studio will never get an exterior design update for at least 10 years
Why does it need one? It's a box to hold the logic board, fans, PSU and sockets together. The only possible fault is that it maybe needs a plastic panel in the lid to improve WiFi/Bluetooth - I don't think that's gonna lead to a rush to upgrade. It's different with laptops where there are displays, speakers mics to upgrade and a pressure to make them as thin and light as the technology will bear - especially the low-end models which sell as fashion items to back-to-school kids.
Of course the fact that there's no interconnect on the M3 Max might be the reason there's no M3 Ultra.
More to the point, if Apple planned the M3 series as a stopgap annual upgrade for the main Mac cash cows - the MBA and MBP - and never planned to make a M3 Ultra there was no
point in including the interconnect on the M3 Max.
Why is the conclusion here that M3 Ultra will be a single die?
Speculation. It's looking more likely that there simply won't be a M3 Ultra - could be wrong, but if the rumours of a rapid across-the-range M4 roll out within 12 months are true it's getting a bit late to release a M3 Ultra.
What is
suggestive is that Apple changed the system with the M3 range - with M1/M2 the Pro, Max and Ultra were all permutations on the same die design - the "Pro" was 2/3 of a "Max" die (even called "chop" during development) and the Ultra was 2x Max dies. That's clearly not the case with the M3 Pro and Max having totally different dies, so really all bets are off for the M3 Ultra.
Meanwhile, although all the M3s are still better than their M2 counterparts but,
relative to each other, the Pro has become slightly less powerful - and more focussed on power efficiency for laptops (which takes off some of its shine for desktop machines) - and the Max has become significantly more powerful all-round, stealing ground from the M2 Ultra.
So, really, the desktop range is ripe for a shake-up - anything is possible: the Studio could get dropped if the Mini can cope with a M3/M4 Max, the Studio Ultra could get dropped if the Studio Max is now good enough, the Mac Pro could
very easily be left to languish as-is (just like the last two Mac Pros and the iMac Pro)... or may be the M3/M4 "Ultra" could turn out to be a "scalable" die like the M1/M2 Max with 1x, 2x and 4x (or Grace/Hopper-like CPU die+GPU/AI) combos showing up in future Studio, Mac Pro and Hypothetical AI server machines... its fun to speculate (disappointment is the most likely outcome).