The Mac Pro exists to grow its capabilities over its owner-lifespan in terms of memory and storage, period. The fact that the current M-series don't support user-upgradable ram is no more of an indication the Mac Pro won't, than the lack of user-upgradable ram on the Intel Macbook series was.
Actually not. The Intel mobile GPUs in those Macs were perfectly capable of dealing with standard DIMMs. In fact, other system vendors use those exact same CPU to provision so-DIMMs on their laptop while Apple solders them down. It is an option that Intel designed into their solution. In part, because the memory controller design is shared between desktop and mobile CPU variants ( either in total or in very major implementation overlap) .
Apple M1's eight , 16-bit LPDDR4 memory controllers are custom. There pretty good chance they are design to only work with the semi-custom LPDDR4 packages that Apple specs friom one . or two, memory contractors to build. Those packages are always placed a fixed distance away for every SoC deployment.
Intel doesn't do that at all. They doesn't do memory controllers for RAM modules that only 1-2 vendors can buy/provision. it is a generic system building component so they build for a wider set of standards.
Apple may not be doing extensive validation of multiple RAM vendors and DIMM packagers. In fact, it is cheaper for them if they do not. It is also cheaper for Apple is they use their hyper custom RAM in as many systems as possible to use economy of scale to get the price down. ( if buy in quantiles of 10M or 100M units then prices get more reasonable. "churn out a bunch of stuff all year long for Apple"... sure there is a discount for that. )
I wouldn't bet the farm on them doing generate DIMMs if they have a "chopped down" memory controller in their systems. Even more so now now that the iMac 24" rolled out with an M1. Pretty than decent chance that the iMac large screen rolls out with something close to the MBP 16" SoC. If Apple does that then prospects for a Mac Pro SoC that has DIMMs is dimmer . There is no volume left for a 'forked' design.
If the iMac and upper spectrum Mini are on an optionally use DIMM path then I would be least skeptical. If the Jade2C and Jade4C bigger M-series show up and have non-homogenous, but still Unified , memory ( mix of LPDDR4 and GDDR6 ) that may open door too. On the other hand, if Apple "thins out " the iMac 27" as much as they did the iMac 24" ... that's a bad sign (i.e. the large screen becomes the "iMac Pro" like. ). If the iMac 27" still has DIMMs that's a good sign.
While Apple is super laser focused on homogenous, unified RAM ( and iGPUs everywhere. and yes that means Mac Pro too), DIMMs may be on a slippery slope.
Apple commiting to extending a Intel Mac Pro shipping as "new" out past end of 2022 that would be another indicator of that is a contributing reason of why it is being kept around. (because the "half sized" one is missing some functionality. Max memory capacity doesn't scale vey high (caps out around 128GB and completely skip ECC support as well as wider 3rd party vendor testing ) ).
There are lots of "dot the i and cross the t" things that AMD/Intel do for general system building support that Apple just may forego. Nobody else is buying Apple SoCs. If they think they don't need it, they don't have to add it in to their designs.
2 to 4 M-series iterations out perhaps they'll change their mind. ( akin to 2013-2019 Mac Pro Rip van Winkle slumber)
Mac Pro 2019 was always quite likely to use some Xeon W series solution. Those all support DIMMs so it didn't make much sense that Apple would avoid them in a large enclosure. Or support more than a couple of slots. Or being able to natively boot Windows. Not a shocker for those because a baseline for most of the Xeon W systems deployed.
Apple has zero track record of delivery a SoC of that type. None. They've put over half of the yearly Mac unit volume onto a single SoC ( M1).
