Only because v3 doesn't provide that much of a boost in numbers over v2, and by the time Apple got a nMP rev ready the chip would be showing its age.
v3 showing its age? It isn't even released yet. Even if Apple rolled out in Jan-Feb ( a couple months after HP/Dell/Lenovo make their moves) it would likely still be another 6-9 months before v4 showed up.
v3 doesn't have to be hugely faster than v2. It primarily has to to fast enough for v0 (old 3600/5600 and previous) and v1 users ( of which none have Macs now) to want to move. This whole notion that n+1 have to completely obsolete the immediately previous version is deeply flawed. Most folks upgrading are moving from substantially older stuff ( 3-7 year old machines) or substantially slower and/or less capable systems ( mini or iMac ) if v2 wasn't "faster enough" to make them move earlier (e.g., the "my Mac Pro 2007-8 is fast enough" crowd ) then incrementally faster v3 will lure more folks than v2 did. It doesn't have to lure
everybody, just enough for a successful sales run.
If Apple doesn't launch until late Spring '15 or Summer '15 the screw up there is Apple not "v3 isn't fast enough". It is far more Apple has too few folks assigned to do the work. There is no revolution in board/system design they need to do to roll out an updated Mac Pro. If they has some seriously lame "the dog ate my homework" excuse as to why they couldn't finish before Summer '15 then yeah sure since they would have blown it might as well slide into late Fall '15.
DDR4 probably needs some lay out tweaks. If they want to use the chipset's USB 3.0 then there is more routing from backplane board to I/O board. The GPUs are AMD with about the same amount of VRAM aren't a huge leap and if go to denser VRAM packages isn't that huge to do an increase. In short there is work to be done, but nothing here is a multi year project.
If Apple wrangled a semi-custom chipset out of Intel (e.g, dumped the SATA lanes they have zero use for and add some a x4 PCIe bundle for a second SSD) that might be a delay that is largely on Intel's plate. But if Apple is using standard Intel parts then they have had all the parts to do an update for a very long time. Other that not working on it there really isn't a blocker that should greatly delay them.
I expect Apple to be a little late.
First, they probably were juggling fixing late beta defects with working on next get prep last Summer/Fall. After Mac Pro came out of "black project stealth mode" the addition users extremely likely found overlooked bugs that needed fixes by launch (or shortly after launch). The MP 2013 almost immediately got a firmware update release largely supports that. Most of 2013 R&D time got spent on the 2013 Mac Pro , not the next one. All the triage probably caused some slide ( not enough to completely stall next gen work, but enough put hiccups in the schedule. )
Apple has a problem in that their competitors didn't skip v0 (Sandy Bridge). For the competitors all of their v2 release work was just minor tweaking of an existing design. That means they could spend most of 2013 doing v3 design work while Apple was still trying to get their v2 system release out the door. Apple isn't likely to catch up until they too can do a intra tick-tock cycle tweak-and-release update. That should come between v3 and v4 (if GPU or other chips release schedules don't get in the way).
Second, the GPUs + drivers are probably finishing later than the Xeon E5 v3 release. AMD only finished, released the non custom FirePros recently. The OS X drivers probably are lower priority than the Windows/Linux drivers those cards needed to launch with. Similarly Apple probably mutates off of AMD reference design which also probably wasn't done over a year ago. I doubt Apple's custom designs are ever going to come "Just as fast" as the non custom stuff that AMD (or Nvidia if they let Apple do custom "pro" cards) does. Nvidia didn't finish their Pro cards any faster so even if did switch probably is same situation.
There is no "wait for Thunderbolt 2 to go into volume production" problem this year, but the non Intel logistics isn't completely clean either.
