This is a market in which a $5k 10 core PC is considered an entry level machine. 3D houses and some music studios have racks of server-grade hardware that is far outside the reach of most mortals.
There is a bigger market of users who like to label themselves pro, but are actually prosumer. It includes smaller music studios (home or not), design studios, serious hobbyists and dabblers, smaller video houses, academic music composers, and so on.
Also many software developers.
These users are not in the Hollywood league, and they typically have a limited budget.
The old cheesegrater MacPro was a good fit for this market. It was also good enough, and expandable enough, to get some interest from the no-compromise pros.
The new MacPro is too expensive for the prosumers, and poor value - and underpowered - for the no-compromise pros.
I'm not yet convinced Apple understands this. Jobs understood it because his links with Pixar meant he could see first hand what Hollywood pros were using.
I don't think the current management is as open to creative input from creative professionals at all levels.
If I'm right, the 2018 Mac Pros will be open like PCs, but they still won't truly satisfy either type of user.