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It really shouldn’t take this long to produce a Pro desktop, so we can only assume Apple is trying to reinvent the wheel yet again.
That isn't the only reasonable assumption. If apply
Occam's razor to the situation the simplest answer as to why there is currently no Mac Pro upgrade available is because they did not work on one. No work, no upgrade.
Presuming that it is some extremely complicated , Area 51 technology that takes 3-4 years to work out isn't particularly likely. There are no need iPod Touches because no one is working on them for the last 2-3 years. Mac Pro was extremely likely in a similar state for at least a couple of years.
Probably, Apple exited the feedback phase on Mac Pro 2013 and for the most part did the iMac Pro. They probably knew that wasn't "everybody" from the Mac Pro target user base but it was some (and aligned with customer trends they had seen; not were 'everyone' was in 2006-2009, but where substantive number of people were going in the future. )
Once the iMac Pro was close to wrapping up into a new product, they decided that there was enough left over to do another Mac Pro ( a different enough demographic that was still outside the scope). Probably still not a super high priority project but assign a bit more than approximately zero resources. ( About April 2017 they knew there were about done with iMac Pro and Mac Pro had a very immediate "image" problem. They needed a stop gap so lower the price and starting hinting they were going to work on a new Mac Pro. )
If they didn't start real scale up on work until mid-2017 it is still in scope of a "greenfield" project to be still in process of wrapping up something ( no Area 51 technology needed. )
Time will tell if they are going to succeed.
Apple as a whole will probably succeed. In the narrow context of having a viable Mac Pro product, time isn't a asset at this point.
They don't really have relatively large amounts of time to 'blow' at this point. The longer it takes them to produce a new Mac Pro at this point the more likely it will not succeed. It is not just a product that Apple needs to produce to be successful in this space. Apple needs to generate some trust. If April 2019 comes and there is a
third "dog ate my homework" moment that trust will be lost to a growing ( and probably substantial ) number of potential buyers. If they did a fourth one in April 2020 that pool of "walk aways" would just get bigger. 2021 .... even bigger. Apple is pragmatically actively engaged in digging a hole at this point.
They will also need a decent match to the value proposition that most of the buyers are looking for ( some open PCI-e slots , more than one internal storage option, etc. ). That part we'll get more information on over time. However, there is no "Wow Area 51 tech. Amazing" dog and pony show they can do that will get them out of that trust hole if they dig too deep. That's just delusional.
18-24 months ( April 2017 -> April 2019 ) is plenty of time for an adequately resourced team (with high talent ) to get a new Mac Pro done. ( at least to the point were primarily finished engineering test and are working out minor kinds on production test. ..... i.e., something creditable to show. )