Apple doesn't want to be in the business of catering to niche professionals which is a difficult market and why they have precisely moving away from that for decades at this point. You don't obtain the top marketcap in the world by serving professionals who need pcie expansion.
Apple could drop both the Mac Pro
and Mac Studio and still have a top 5 market cap. It has extremely little to do with PCI-e slots.
The major crux of the issue is that the Mac Pro (at least since the Intel era when this general market segment product got that name) has been powered by a server focus chip that was tossed into the Personal computer market as a secondary, side-effect. The server deployments primarily paid for the chip development. The workstation application was just a 'side show'.
Intel , AMD , Nvidia... they don't kill off high end chips in a year. Even if they might come out with something new they'd contine to sell the older ones (at lower volumes).
The basic Mx chip is shared with the iPad. The major problem for the Mac Pro 'bigger than Ultra' SoC is that it has no 'hand me down' product to go into. The M2 went Mac , iPad Pro , iPad Air. Similar on the iPhone side. The Axx chips trickle down into iPads , AppleTV etc. The watch chips trickle down into Home Pods . etc.
So one major problem here is that if give Mac Pro a chip that Mac STudio doesn't have where does it go?
[ There is lots of handwaving in some corners to the "Apple AI server". But do they really want to deliver 'hand me down' product there? Also a service that does't generate revenue. No revenue bigger driver looking for a 'cheaper' solution as opposed to the most expensive option. ]
At this point the Mac Pro for Apple is more of a halo product that every once in a while they can show it off and it's good for branding to still have some presence for that segment of users.
The Mac Pro is more a 'hobby' product than a halo product. 2013 -> 2019 2019 -> 2023 ... that isn't 'halo'. That is do it occasionally when get some spare time.
Issue now is that Apple Silicone has really made it difficult to differentiate this especially with a Mac Studio in the mix. I would be willing to bet Apple would love to just kill off the current Mac Pro and rename the Mac Studio as the Mac Pro(similar to what they did back in 2013).
More so Apple would like to put the performance of the legacy Mac Pro into a MBP 16" and folks just buy that.
The Studio is handy to do , but the bulk of the Mac line up is laptops. Apple is trying to make best laptop possible. The desktop line up somewhat just falls out of that as a side effect.
The question becomes
1. does Apple just keep the Mac Pro the same as it is....basically keep it as the niche Mac Studio Tower
2. do they kill it off at some point and Mac Studio and they merge?
3. does Apple pull the halo card and at WWDC they unveil some kind of Mac Pro only monolithic chip or control die multiple GPU/CPU dies that grabs headlines and makes its rounds in the mainstream media.
Option 1 is most likely I would think.
Mac Pro isn't as much of a Mac studio tower as it could be because there is no Mn Max option Mac Pro.
The Mac Pro is segmented off substantially more than just "more slots". [ Apple shifted the MP 2019 up 100% for entry price $3K -> $6K. ]
The more major issue is I/O differentiation isn't as large as it could be. It isn't that it is a "tower" format .
Folks can stuff a Mini or Studio into a xMac enclosure.
3U enclosure to install and secure one Mac Studio with optional PCIe card expansion in a standard 19-inch rack.
www.sonnettech.com
If Apple a "Mac on a card" option that slotted into a Mac Pro that would be a larger value add than the "tower" aspect. What is missing is something that could be added that some "scale up" heft to the box. Use the PCI-e connection to very fast network the instances together and have a cluster in a box.
As long as the Mac Pro is mainly aiming at the very high priced A/V card bought years and years ago that don't want to replace then it has major issues keeping folks happy who are outside that market.
Defacto they already have option 3. There is silicon (and connectors) in the Ultra that the Studio can't physically use. It is far I/O as opposed to segmenting off the CPU/GPU cores, but can't get to it on Studio.
What Apple did with M2 Ultra was a bit asymmetric. x8 and x16 PCI-e v4. two x16 PCI-e v4 or two x8 PCI-e v5 would be a substantive improvement. Adds some better software driver and/or direct I/O virtualization support and the Mac Pro would be on much better footing to a better value proposition.