As usual your analysis is spot-on technically.
I would like you to be right about how it all turns out, but I think you won't be, because I'm dubious about how much Apple will be willing to invest in chiplet designs for high-end machines. The pros are a tiny part of their revenue stream. If they were willing to do a truly custom design for the Mac Pro, then I think you'd be right on the money.
It doesn't have to be just for highest end. If dribbled down to the Mac Studio and perhaps a large screen iMac there is would be a wider base to spread costs over. Part of problem is the cap on number of Mac desktop models ( to get Studio had to kill the large screen iMac. As long as have that kind of dynamic around there is always going to not a big enough desktop user space to support mild derivate alternatives. If the MBP 13 stays around after the MBA 16 comes also shows how arbitrary the cap on desktop Mac models is. ). But yes, there is some likelihood that Apple will shave costs that will hurt the Mac Pro's competitiveness in general workstation market.
Apple could push the M3 Max to have 16 cores ( two 4 core P clusters and two 4 core E clusters). Apple could cap out an Ultra with 32 cores and just give away more of the old Mac Pro user space. So 32 , not 40, and half of them E cores. Throw an I/O chiplet between the M3 Max dies and spawn a small amount of PCI-e provisioning to be backhaul for some PCI-e slots and just stop.
There might be a Max-Plus die that tossed in more GPU cores (and memory controllers) and kept the CPU count the same. And then pair those. Kind of doubling down on the "too chunky" chiplet notion to go even more chunky.
(i.e., whatever shrinkage N3/N2/etc provide just keep same M2 Max die size and just stuff more in there. ) . However, still giving up on the > 2 die options. Also gives them good excuse to avoid implementing ECC on Memory because Max capacity is going to be limited.
If Apple is going that route I think there is a way to expand the revenue stream by putting some larger M-series on a PCI-e card and selling "Mac on Card" options for the folks who are just going to leave go to an x86 container workstation. 400-600W GPU cards provisioning opens the door just sticking a whole computer on a PCI-e card and running a separate OS instance on the card. However, that requires Apple to think outside the box to expand into new markets.
There is one possibility though that might push things in favor of your scenario. If they decide they need massively wide CPU/GPU for their xR headset line (future generations, if not the first) then they might see investment in a real chiplet design as paying off for xR devices, not just Mac Pros.
Reportedly the higher end headset has two processor packages. That actually makes sense given the limited enclosure. That would be somewhat opposite of chiplets though since spreading them out to fit on either side of the two eyes. Even if they did it would only be a pairing of just two heterogeneous chips (one UltraFusion connector to rule them all ... still )
The hurdle for the xr headset is that there is lots of input coming in. 12+ cameras and inferencing on that raw data that really , really , really doesn't need to be shared with mostly everything else (besides mirroring for the screens) except for the relatively much smaller results.
Similar with the iPhone once Apple gets a modem deploy. Base A-series die with one , and only one, modem chiplet. ( Fab process wise may not make sense to make them use exactly the same stuff.)
Hopefully we'll get some clarity at WWDC. I'm pretty disappointed we didn't get first M3s already by now, though I won't be truly surprised by them not showing up unless they're still MIA in the fall.
If WWDC reveals that Apple has thrown a collectively huge amount of transistors at the headset's processing abilities. Just how custom the design is could be a factor in why don't want to put highly custom other dies into the design workload queue.
I thought that Apple would have used TSMC N3 to solve some of the Mac Pro 'problems' in matching performance and shown something by now. But looks like that they are not being that aggressive. I would be surprised if Apple wanted to build a notion that the M-series was going to refresh on some rigid 12 month cadence. It works for the iPhone because the iPhone generates huge revenues (
and sells one , two year old stuff as new. Millions and millions of 'hand me down' models to sell into.).
So a >12 month cadence isn't surprising. I though the Mac Pro might get corner case because haven't even started yet. They also are not generating good expectation by being grossly late to even start.
Won't be surprised if the iMac 24" gets M3 first. That on some 'wish thinking' plan worked out years ago the M3 iMac would have arrived around the same time as the 25th anniversary of the first iMac. N3 production slid out about 6 months from the most optimistic roadmap back then and so that iMac slides back about 6 months. But fixed in stone A-series launch dates and even longer inventory build production times for N3 messed that plan up. There was no pre demand bubble inventory slot left to get modest volumes out in the Spring. iMac isn't strategic product now so slide doesn't matter.