If you're referring to the 5,1 model, I believe the only reason there was a 2012 refresh with 'newer' CPUs using the same socket was that the 2010 model Nehalem CPUs were about to be discontinued by Intel. 2012 was considered a speed bump at the time but Apple were forced to act by something out of their control.
Forced to act, but not because of those specific 2010 CPUs. The 2012 MP stuff was 2009-2010 based too. The 3530 used in the baseline Mac Pro 2010 was retired for try buys until 2014. The retail was September 2013.
http://www.cpu-world.com/CPUs/Xeon/Intel-Xeon W3530 - AT80601000897AB (BX80601W3530).html
The 2012 Mac Pro's 3465 entry processor.
Exact same retirement dates.
http://www.cpu-world.com/CPUs/Xeon/Intel-Xeon W3565 - AT80601002727AB (BX80601W3565).html
In fact the 3565 has an even
older introduction date than the 3530.
What happened in June 2012 that forced Apple to act was that
all of their workstation competitors introduced Xeon E5 1600 v1 ( Sandy Bridge ) systems and Apple had nothing to ship in the same class. There were processors that Apple hadn't used into the 2010 models ( and price cuts to be funneled out) and Apple was forced to put on some externally visible show they were not completely abandoning the Mac Pro.
There certainly was newer, better GPUs and Apple did nothing with those. Again the GPUs they stuck with were in same slippery slope of not that far from discontinuation as the CPUs. [ A third party card came that could have offered as a standard build but didn't. ]
And finally, we have the evolution of the METAL API which replaces OpenCL which was the big thing that Apple was trying to push when they launched the 6,1. Crucially, this is billed as Apple's own version of DirectX so they are at least taking control of their own graphics destiny rather than relying on ancient implementations of OpenGL or OpenCL.
Metal doesn't really replace OpenCL any more than Vulkan replaces OpenCL. There are some computational overlaps, but it isn't a replacement.
Just make sure the cooling solution can deal with an Nvidia 1080Ti or an AMD VEGA and Apple can set about designing a silent case around a single GPU and maybe a set of E3 Xeon CPUs to keep the hackintoshers at bay, all the while getting Nvidia or AMD to create Mac Editions of their hardware built to fit the slot (custom size if need be) and also help prevent people from flashing Mac ROMS onto a variety of non standard hardware.
The core issue is the standard form PCI-e cards don't deal with Thunderbolt well. A secondary, optional, GPU card doesn't have to tie into Thunderbolt. However, the primary GPU card does have that constraint. A single secondary, optional slot still allows those who want some 3rd party GPU to have one. So would have two GPUs. If want to ignore one than can.
Mac Pro's are in the 1-3% of Mac market zone. Getting along with the other 97+ % of the Mac market supersedes some smaller subset of 1-3% wanting to throw TB completely off the Mac Pro.
Using E3 doesn't make much sense either. The RAM and core count limitations don't help much. The PCI-e limitations makes it a non starter for a Mac Pro. Skylake-W may have some E3 like limits on the 4 core option (if the uncore PCI-e limitations are expanded out to make the rest of the line up), but at least it will be a shared socket and chipset design. A forked logic board likely doesn't make sense. It isn't going to save that much money in Apple's context of limited R&D resources. . I don't think cost differences are gong to be major difference here. ( the baseline E3 die that Intel stuffs into a Skylake-W socket probably won't be that much more expensive even if adds more PCI-e lanes. If price is the same because the lane limitations are same, then it is still a non starter. )
The Hackintosh folks aren't really a problem. They market impact is likely in the sub 1% range. They moan and groan extremely loudly but the dollars they represent probably isn't worth chasing after. Trying to get that down as close to zero as possible has diminishing returns financially.
One, secondary GPU slot would likely cut way down on the moan and groan. Highly likely enough to keep that Hackintosh down in the sub 1% zone. Some folks are going to complain no matter what.
What I think Apple will be taking their time over is making a small case that will be as silent or quiet as the 2013 Mac Pro while being able to continue fitting on the desktop and they may find themselves in competition with a lot of vendors if that's all they were aiming for. It's also Apple so we should expect more innovation than that - just don't involve dousing modules in mineral oil or something equally as exotic
.
Desktop targeted doesn't mean it is an Xmac. And when doesn't Apple has lots of competition across the whole Mac line up. Apple has about 7% of traditional form factors PC market. On any given day 90+ % of folks are buying something other than macOS. That is a wake up and "same stuff, different day" factor for Apple executives. It isn't new, nor is it likely to change in the future.
