The 2019 Mac Pro isn’t targeted at the home/consumer market. It’s mostly for business/corporate use, and pros who use it to generate revenue. It covers the low, mid and high range of requirements for Apple customers who want/need an expandable tower instead of an all-in-one like iMac Pro.
Your mileage for "low range" may vary.
For us "old timers", the desire for "affordable expandable desktop" might come from the era 25-15 years ago, when suddenly the digitalization democratized making "pro grade" video.
Price for the tools dropped like one tenth for what it used to be. And Apple was in the middle of this all. G5 was supercomputer for all. Just like they advertised it. MP was even better.
In broader picture, I think, this goes in line with economical evolvement in general.
Segregation of wealth is in the air.
In most most visual content making centers around the globe, even the prices of work room have risen so high that the market has split a great deal.
Those who break to the success, already spent so much money that the price for tools Apple is offering them, does not matter.
Apple has no intent to offer something "optimal" for those who don't break to success.
Also, Macs in general don't matter economically to Apple much. Apple does not have to offer anything "optimal" for small timers, since especially non AIO-dekstops are just for the show. They don't matter in revenues or profits of Apple. Jobs might have some ideological reasons for making "middle macs", but surely Cook does not. I'd guess that Cook just don't want Apple to waste any more resouces for developing mac models than it has to. They needed a flagship and they got it. Why offer something in between? (tautological) Also, Apple tries to keep the illusion alive, that iPad is all you need for "middle ground" creative work. They can't break this idealogy for offering a "pc" in post-pc era.
The question arised here and many more times is, that why do Apple focus on AIO?
I'd like to hear opinions about this.
One answer would be, that because that is (for the long time, in the past & future) the most selling model, they want to focus on that, just because they want to have as limited range of models as possible.
Second thing would be, that they get the components for imac for so low price, that it would be economically just bad business to do something else.
My gues still is, that the answer to this is support costs. With AIO with as less as possible, what user can change, the less they need expensive support for that mac.
With expandable mac Apple faces lots of problems with compatibility and drivers, their own and 3rd pary. This might also be one of the reasons why they break up witn nVidia. Lots of problems with drivers and pointing fingers who's fault is that. As much as they keep the driver developement within the company, for all that limited amount of drivers they need to produce, there's less of hassle, hence less the costs.
Anecdote: I bought hp's eGPU for mini2012. Turns out that it works only with TB3, nevermind what egpu.io says. Nobody takes responsible about the compatability, even TB should be a "standard". Same thing with LG's 5k2k (tb3) display & mini2018. Doesn't work, but nobody is responsible.
As long as mac's market share is in one digit and and expendable desktop mac's market share is fraction of a per mil, there won't be economical interest to get any pcie-card working in any mac.
Just as a side note, it's pretty tragicomic to see people selling their few years old soldered imacs in FB and other markets, with prices that used to be normal, when macs were upgradeable. People just don't understand why they macs are not so interesting any more. Most macs might have solid hardware that can last a decade (not counting MBP, which has all battery, gpu, display & keyboard problems imaginable for last 3-5 years), but I can't advice anybody that ask me, to buy a mac with 8GB soldered ram for longer use in the future.
