I believe the Apple Silicon Mac Pro model will have PCIe slots, just fewer. My guess would be four (so half of what MP 7,1 can hold) so that it can handle up to two MPX cards or an MPX card and two single-slot cards.
That is basically a re-defintion of a MPX bay. Full sized MPX modules are 4 slots wide. And there are two sockets per bay and one MPX connector. on the first slot of the bay.
You're pointing at half size MPX modules as being the "norm". That is a huge stretch. Of the five MPX modules Apple has offered so far only two have been half sized ones ( 580X and 5500). One could hand wave and say that they'll be Apple only GPUs and only will need two "height" cooling and still be top end market competitive. Thown in the stoarge MPX modules and the ratio of half : full height modules shrinks even more lopsided.
The MPX connector in the bay is doing bandwidth sharing with the second slot in the bay. Unless doing major upgrades to the M-series SoC can't both share the bandwidth on the second MPX bay slot and use the slot for top end GPU communication bandwidth. That is why "half sized" MPX modules don't come with Thunderbolt ports. They don't pull Thunderbolt provisioning data on the MPX connector. (i.e., if MPX modules 'uncovers' the second slot, it doesn't pull data from the second slot indirectly). You proposal is that Apple completely change that role.
IMHO, I think it is a stretch to think that the M-series SoC used in the Mac Pro is even going to provision more than PCI-e v3. Even if Apple did upgrade to PCI-e v4 in the second generation , likely not going to get that many lanes. The slot cut is more likely driven because the aggregate I/O bandwdith just isn't there. Something like:
One MPX Bay ( two slot, quad wide , one aux power connector. )
One x16 electrical slot single width ( one Afterburner card only bus )
On x4 semicustom I/O card. (single width )
If lucky maybe one more x8 card single with ( current slot 6 ) , but decent chance this bandwidth get consumed by dual 10GbE , some discrete SATA controller for at least same two ports as have now (maybe give out 3 (or 4 ) and allow for incrementally taller bracket). WiFi/Bluetooth and misc.
Apple spent a mint to bring Mac Pro 7,1 to market. Customers then spent a mint to purchase them. Neither Apple nor the customers are going to want MP 7,1 to be a "one and done" model like MP 6,1 was and Apple clearly engineered MP 7,1 with sufficient overhead to take successor generations of CPUs and GPUs.
Apple probably hit break even on Mac Pro 2019 development costs by June of this year (if not sooner). There is a 20+ % markup on every screw, wire, washer, NAND chip , etc. in that system with a "low volume " tax on the overall system. Apple picked the most expensive > 16 core options they could and taxed those high markups also. The longer it takes an W-3300 Ice Lake to come out ( e.g., slides to 2022 ) the more likely Apple has hit breakeven.
There are no other substantive "loss leaders" in the rest of the Mac line up . So not like DEll/HP/Lenovo where have to use workstation profits to cover losses in other parts of the business. If don't have to "rob Peter to pay Paul" the path to breakeven is much quicker because can use the full profits to pay off the upfront costs.
It is more like it is relative cheap for Apple to do an W-3300 update to a W-3200 system. Keep the same already paid for chassis design. Keep the same fans and cooling set up that is already paid for. Cheap. Same DIMMS slots with a slightly different traces back to the CPU. relatively cheap. Same 68 PCI-e lanes coming out of the socket. Relatively cheap ( need some re-timers for PCI-e v4 run over a distance , but not a moon shot project). If they only care about increasing slot 1 and slot 3 to PCI-e v4, then they could keep same PLEX PCI-e swtich (and rest of the slots all stay the same). Cheap, pratically already paid for R&D. Same power supply. Paid for.
Apple is probably doing this not because they are deep in the hole in R&D cost recovering. It is likely because it is vastly cheaper than coming up with a relative super low volume SoC that can actually compete with W-3300 level aggregate bandwidth. Apple can skip that since they don't have it (and probably don't want it.) . More likely Apple wants to keep the iGPU feed with data and trading off more higher aggregate memory I/O bandwidth for lower more generic PCI-e aggregate bandwidth (and power consumption) out of the SoC.
Also, decent chance Apple started on a W-3300 iMac Pro and pulled the plug in it when final W-3300 thermals blew past limits and slid out from 2020 to 2022. Again this is chance to re-use initial work already paid for ( or in this case already invested in).
Releasing a half sized , M-series Mac Pro will probably cut into profits that the full sized Mac Pro generates. Only deep slot count lovers and higher core count with high RAM working set footprint folks will be drawn to it. But if mostly already paid for foundation then breakeven doesn't need as many even higher priced units to sell . ( plus redoing the system allows Apple to do a face saving price cut so that the systems are more competitive. W-3300 will have incrementally lower prices. And substantively new lower prices at the top end where Intel dropped the extremely heavy "> 1TB " RAM tax . )
The only drawback for existing MP 7,1 owners is that W-3300 will use a new socket and therefore will require a new systemboard so a "drop-in" CPU upgrade will not be possible.
If the MP 8,1 can easily take the $5K sunk cost Vega MPX modules that's just fine. Some folks spend far more money on what goes in the PCI-e slots than what goes in the CPU socket. that is somewhat reflexive also. New 6000 series MPX that costs $2.5K is probably bigger draw for many 7.1 owners . The CPU socket is secondary as it isn't the computational chokepoint.
The W-3300 is yet another "dead ender" CPU socket. The "I gotta get more than one CPU generation in my socket" folks aren't going to be entirely happy with either one.
Both the W-3200 and W-3300 will get cheaper over an extended period of time to "drop in" higher core counts later. The W-3300 can probalby take old Xeon SP gen 3 also as they fill up the used sales in several years time.
