I rather think that the iMac Pro was the intended replacement for the Trashcan Mac Pro but something happened that made them change their mind...twitter outcry from pros on Twitter, VR/AR who knows?
It is extremely unlikely it was social media (Twitter, these forums , etc). The April 2017 meeting/"pow wow" they had with reporters about the "Macs and the Pro space" was 2-3 months before the iMac Pro was introduced. Stories about the Mac Pro 2013 being going so long without update was an issue they were trying to control though. I wasn't a coincidence that the MP 2013 got a price cut around the same time as that meeting.
There were about 1-2 odd threads with odd matching of iMac with "Xeon" in these forum that got some attention, but not much.
AR ( augmented reality ) is a very unlikely primary driver. Apple has deployed AR apps on the iPhone. Ultra-extreme VR is trendy in the press but not a substantive base. The broader category of GPU computational 'horsepower' was probably an issue though. The iMac Pro would end up with essentially the same power limitations at the MP 2013 had. Where two GPUs was useful ( and there were MP 2013 customers happy in that camp) the 400-480W envelope of the iMac Pro (MP 2013) doesn't work as well. That isn't just "VR" but more computational horsepower for a variety of solutions. For example doubling that to 800-950W would capture another class of solutions.
They likely did get some feedback from the extremely small number of non disclosure customers they briefed on it. However, the notion that they completely didn't know they were 'missing' some of the former customer base with the MP 2013 and iMac Pro ... it is highly unlikely that happened. The number of folks who were still sticking around on 5,1's probably got clearer. They also probably tweaked some of the 27" iMac and MP 2013 users with the whole no RAM door thing too.
The other problem was component suppliers. AMD drifting off their long term GPU roadmap milestones was a problem for the MP 2013. Intel getting twisted up later was only yet another example. Apple needs a system where they make some high percentage of the design calls but has some flexibility when their projections of where things are gong is off (can't see 100% of the future). Thunderbolt gives them an out, but a couple of standard slots would be an even broader out if added on top of that baseline. I doubt they changed their minds on Thunderbolt. Just more pragmatic about what the scope of that is .
What they may have changed their mind on is just how small a percentage can they go. Is 2, 1 , or 0.5% of the Mac market worth being in. The iMac Pro really doesn't change that much.
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And the belief that Apple's going to replace intel with ARM? It doesn't make sense. Apple knows their creative base, (a big chunk of who these machines appeal to), rely on programs like Logic, Adobe CC, Pro Tools etc... None of which are ready to run a full suite on iOS/ARM.
The full Adobe CC suite? No. Photoshop? already announced as coming in 2019.
An ARM SoC into the current MacBook shell would make sense. iPad Pro getting a USB Type C connector ( one and only one connector). MacBook with one and only one USB Type-C connector. They are about completely matched up. Apple has done the viability of a single general data port device. Rip the headphone jack off and put on a FaceID camera array and they'd be completely lined up.
Apple with an iOS iBook that went after the Chromebook and the sub $900 Windows 2-in-1s makes sense. Apple expanding "iPad iOS" into new spaces wouldn't be a surprise. They could use the shell of a Mac to go that way. ( some chromebooks and windows 2-in-1 largely share the almost the exact same chassis. )
The entire Mac line up top-to-bottom on ARM is lunacy. Apple splitting the Mac line up into two "halves" is highly dubious, but Apple has done some dubious things.
(Although Logic technically could, Apple are well aware that Logic users love their 3rd party AUs.) Switching everything else over to ARM however ....
Plus, why would they have just introduced the introduce the iMac Pro with Intel, only to overshadow it with a machine that appeals to the same exact sector but runs only on Arm? That just doesn't make sense...
I think there is a difference between the subset of the Mac market that the "industry analysts" care about and the whole Mac Market. It think they are looking for Apple to largely go 'laptop only" and shift to ARM. A mobile Mac skew of the world. That is most of the revenues and units shipments so that is all the "matters' ( to folks picking stock prices ).
If Apple was shifting everything over to ARM as the fantasy outlines, then the iMac Pro would shift over too. The new Mac Pro with more than one internal drive (some M.2 sockets) , a couple of standard slots , a substantially higher power supply and cooling budget, twice as many RAM DIMM slots, etc. wouldn't be "the same exact sector" even if keep the CPUs and GPUs largely the same ( but run in different thermal envelopes).