That "up to a point" number is likely smaller than 4. Selecting 4 because the initial Mac Pro had 4 is largely form over function inference. Here is a thread with a survey ( which probably has some demographic sampling issues but it is better than "old one had for so that is the right number" hand waving.)
https://forums.macrumors.com/threads/whats-occupying-your-pci-e-slots.1614365/
1. There is are enough empty slots there to illustration that average is not going to be between 3.5-4. So it is lower than 4. .
2. A common occurrence is a "boot" GPU. A GPU card that is in the mix of some directly (or indirectly snarfed from Apple funded work) card that user. In other words there is a default display card. So pragmatically 4-1 = 3 . That slot isn't for 'random'/arbitrary cards. Helping to fill that is a role that Apple has to play a part in. if Apple doesn't diligently play that role then the overall product has a major problem .
3. Even in the older Mac Pros it is split 2 x16 and 2 x4 . So lots of folks got buy with just one x16 slot so x8-x16 needs in the old system. That wouldn't change for a new one.
4. Thunderbolt v3 largely covers the bandwidth and the latency of a x4 slot. If a x4 standard slot is "covered up" by TBv3 they user can still get to the bandwidth. So going back to the survey, there are more a few examples of USB 3.0 ( some 3.1 ) cards stuffed into a x4 slot. USB 3.2 , 3.4 , 4.0 , and probably 5.0 probably can all be covered by a TBv3 . The function ( not the form) of keeping up with the Jones in USB ports probably would be fine for the next 8-9 years with a TBv3 port.
If Apple puts 4-6 modern TYPE-A USB 3.1 gen 1 ports on the revised Mac Pro in 2019 that system would have have very good USB coverage forward (and back) for 8-9 service life of that machine. When the PCH chipset bumps on a future iteration to provisiong 4 USB 3.1 gen2 that wouldn't require much change to the design.
One x16 and one x4 slot would cover just everyone on that survey ( without getting into the weeds of folks who need a 'pure' Nvidia GPU line up in their Mac Pro ... i.e., GPU fanboy wars. ). At four slots already on the diminishing returns slide.
From the overall Mac market spective the Mac Pro even when has a healthy machine is probably in the 1-2% range. It will help smooth out the 'slop'/noise in the other vast majority of the Mac market ( units up/down on refresh timings , competitive changes , etc.) but it is highly unlikely to move the needle at all in a significant way. At best it will slightly wiggle less.
If the iMac and/or the 13" laptops all take 2% the Mac Pro isn't going to be able to bend that back up to maintain. The Mac Pro has been largely comatose over the last 8 years. Over that time Mac units have largely outpaced or match the PC market. ( not quirky corner Quarters but overall yearly trends). Mac Pro isn't some hidden super growth submarket that Apple has ignored. The "maintaine or get back to nominal growth" is largely about the Mac Pro submarket itself, not the Mac ecosystem. The Mac Pro could get back to trending water against overall Mac growth if Apple cleaned up their act in this space.
There is a whole segment of Windows users to left windows and came to Macs. There is bleed both ways. Only looking at one direction isn't going result in good market analysis fro the Mac Pro.
There are tons of folks who want an xMac and the Mac Pro isn't going to be an xMac.
Every day 90+% of the classic PC market doesn't buy a Mac. Coming up with folks who don't want to buy a Mac is like talking about how the sky is blue. It was blue yesterday. Is is blue today. It is going to be blue tomorrow.
When has Apple been off of the RAM upgrade in the Mac Pro space???? They were replaceable on the 2013 model. Why wouldn't that be true on the next iteration. That's largely hysteria driven.
Storage. I think Apple is largely on track to equating that with PCI-e. again there is presence in the survey of more a few PCI-e cards to provision SSDs. There is a difference between standard PCI-e slot and a M.2 PCI-e slot. Apple putting deadicated SSD 1-2 PCI-e slots ( m.2 ) in would be a move in the right direction. The function is provision slots for internal SSDs. Saying that has to be in x4 standard PCI-e slot is a form issue.
'Extra' SATA cable flopping around inside the enclosure is different from there being a solution in the storage capacity space inside the machine. ( same issue just because old system had 4 spinner sleds doesn't necessarily mean going to have 4 spinners now. ). What Apple needs to provide is more internal storage capacity than the rest of the Mac line up. That is not necessarily "best NAS box" level of storage.
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The T-x are in the "Pro" Mac products now because it is expensive. As the T-series evolves, it is extremely that it will be rolled out to all Macs. They will all have them. Right now Apple is in a catch-22. They are in so few systems they are probably a little too expensive for the entry level priced Macs. but they need volume to make them more affordable. ( which can't get until roll the T-series out to maximum volume.).
booting and security is a common feature that all Macs needs. That is a huge chunk of what the T-series is about.
It is highly unlikles that small volume run systems like the iMac Pro and Mac Pro are going to get relatively super low run T-series chips. What Apple needs is volume ranges in the 10's of millions ( or maybe multiples of millions) , not 10's of thousands. They can work out the the kinks with these higher margin/profit systems first before while ramp to that that kind of high volume.
The iMacs are probably next ( I"m not so sure about the Mini or any "more affordable' laptop that may come in the Fall).
Sell it with faster RAM, higher real world turbo speeds (better cooling), faster buses, better wifi, higher expanding options, more ports, future upgradeability at the same price or lower (exluding the screen, obviously) and I may consider the same hardware one year later.
More RAM, not faster RAM would be a differentiator. The iMac Pro just has 4 DIMM slots. If the Mac Pro had 8 DIMMs slots that would be a differentiator. Apple would have to get over their mantra of filling all DIMM slots in every configuration (and the all but one exception) . [ Or crank of the base price by putting in eight 8GB DIMMs as the baseline. Have to start at 64GB minimally] The folks who pragmatically want to run large RAM disks is something that the iMac Pro doesn't cover well.
The busses don't have to be faster, but using all (or most of ) them is an issue.
if more ports means "pour on TB ports like ketchup" that is a bad idea. Port placement as much as "more" might help. (e.g, two USB ports on the front of the system. Maybe a SD-Card port on the front. ). Outnumbering the iMac ports isn't so much an issue. That "sit and spin" feature of the Mac Pro 2013 wasn't well motivated. All the more so, if Apple goes back to deskside/underdesk default placement of Mac Pro.
