Looking at the price and lack of maintainability (in terms of upgrades and fixes) of the new iMac 'Pro' makes me think that the primary user base that Apple is targeting with these products are those that like to be called 'pro' (who also have a lot of money - through their dull but highly paid banking jobs) rather than those that actually are.
Not all professional users upgrade or fix their own equipment. That is a subset of the professional user community - the people with the talent and skills to do
everything. However, many others have much more profitable things to do with their time than repair/maintain their equipment. In organizations of any size, that's what the maintenance (or IT) staff is for. In most of the broadcast and recording facilities I worked in, the repair shop (i.e., access to tools) was locked, to prevent the damage that could be done by well-meaning "professionals."
It's little different than in any other profession. Physicians don't modify or maintain their medical electronics. Fleet bus drivers don't change oil or overhaul engines. Airline pilots don't maintain aircraft. Chefs may touch-up their knife blades with a sharpening steel, but they usually send those blades out to professional sharpeners when a few swipes of a steel no longer cuts it. Skilled CGI artists are rarely capable of writing code or repairing a computer.
It seems simple enough to me. Sophisticated tools spawn both professional users
and professional maintainers. Users rarely give a hoot about maintainability/ease of modification. The repair and maintenance staff's convenience takes a back seat to the user's wants and needs. Facility owners like equipment that can do the job without modification/customization, that has a small footprint, and is easily swapped-out with a spare unit if/when repair is necessary (whole-unit replacement). So, if an iMac Pro can do the job right out of the box, at the right price, and will continue to perform satisfactorily for 3-4 years, then they'll buy an iMac Pro.