If you render on CPU like me than the math is simple, a 64 core TR will render almost twice as fast as a 28core Xeon, it also cost twice that much (you can find the 28core M version new on eBay for 2200$).
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There are also people speculating that a new Intel MacPro with latest Xeons is about to be released, for me it’s extremely unlikely that will happen but you never know..
Half the cores for twice the price is the problem the Mac Pro 2019 has. Apple only rarely cuts the price of a system once they have released it. ( The Mac Pro 2013 got a price reduction in 2017 . A reduction but it took 4
years to happen. If Apple kept the Mac Pro 2019 around until 2023 maybe they'd reduce the price.. but before then probably not. ). Apple's "rules" allow them to reprise products when release a new one. Lastest W-3300 would allow Apple to move the 12 , 16 , and 24 core systems to lower price points ( and toss the 8 core model).
The other huge assumption you're making is that the Apple Silicon (AS) Mac Pro ( "8,1" ) will actually be a direct replacement for the 2019 ( 7,1) model. All the evidence so far is that it will not be. Apple has zero support for 3rd GPU cards in macOS on AS. None. That puts multiple high end x16 slots with 300+ W aux power sockets in doubt. No drivers then no need for GPU cards. If continues their "war' against classic discrete GPUs onto the Mac Pro then probably going to loose slots. Also probably going to loose DIMMs also. High end GPUs don't use DIMMs . Apple's high end iGPU probably won't for the same power savings and performance reasons.
AS also is quite unlikely to get to parity with the 64 (and up) core options for Windows. macOS doesn't support more than 64 threads and Apple is quite unlikely to go that way just for one , extremely low volume product. ( on the W-3300 there is pretty good chance they would skip the 38 core option or else lock-it to turn off hypertheads ) .
So the folks who have super insatiable desires for high core counts , the Mac probably isn't going that way. AS is going to a higher amount of more specialized processors ( AMX , Neural cores, GPU cores , image processors , etc.). Those are primairly all called by specialized Apple libraries. Building SoC to brute force a generic "add/sub/mult/div" application code onto triple digits numbers of cores at very high TDPs is not what they are focused on at all. The CPU core count will go up, but not on some unbounded , "first priority" , fast track and probably plateau after the AS transition stabilizes.
Nor is AS out to directly replace 375-425W massive GPU cards with something equally as large and modular.
In short, the assumption that Apple is going to make a "Threadripper killer" SoC is a very big leap lacking in evidence. They probably are not. So if they are not making something to kill of the Threadripper/W-3xxx series then why not do an update?
Apple will probably have something with some substantive overlap ( e.g., max out at 40-50 general application cores ), but over the long term chase AMD/Intel/Ampere into the triple digits .... probably not. High end I/O wise with multiple PCI-e v4 (v5) x16 slots ... again probably not.
On Apple systems with AS the GPU has access to the exact same amount of memory as the CPU has .... what is really the "advantage" the CPU only render is going to bring to the table? It is not a bigger workingspace room. It is not more math function units. It isn't ray tracing specific math function units. It isn't more power efficient.
The A15 is illustrative of all of these Apple focused objectives. CPU core counts same. Most of additional transistor budget allocated to additional GPU core, image processing improvements ( can en/decode ProRes now) , Neural cores , system level cache , etc. the Mac Pro AS will get a bigger overall transitor budget but the weightings are likely to be similar in priority budget assignments.
CPU renderers will be more portable but AS isn't about maximum portiablity. It is about better Macs and iPads first and portability after that.