The truth is that only a very small percentage of buyers ever upgrade their machines. Making them easily upgradable so that the small group can upgrade adds cost that all buyers incur.
Soldering in certain components also means less chance for issues related to such removable pieces.
There are most certainly other reasons the Mac mini isn't easily user upgradable. You're incorrect in your assessment.
Agreed with most of this, save one thing.
SSD.
SSDs are more reliable and robust, and certainly FASTER than HDDs were, but even with TRIM support, there is possibility of drive failure, and that increases with write cycles and age.
I know Apple wants to sell new computers when AppleCare expires, but a computer like this should be viable well after 3 years, if spec'd correctly... and not being able to service the SSD means that it becomes a paperweight without an external drive, if the internal drive were to fail even if the RAM and CPU still function... or somehow becomes a zero-client interface host.
CPU, I can live with buying that up-front, for the reasons previously mentioned in terms of MLB, bus, memory and other architectural considerations that go with the CPU. But a socketed blade SSD would have been nice, along with the DIMMs being replaceable.
That being said... it seems like MacMini's new role appears to be a central processing hub between external components, not really much of an entry-level computer anymore.
An entry level computer now, BTW... is a 400$ iPad, not a desktop anything. Step-up is iPad Pro, MacBook Air, up-level MacMini, or entry iMac for full MacOS functionality... each hovering around ~1000$.
The entry level MacMini looks like a server cluster machine, or a MacOS host for the most basic functions, or the furthest from a power user possible, almost like a Kiosk installation.
up-level MacMini is workable for something the iMac, and iMac Pro can't quite as well... integrate into a multi-monitor workstation... and the MacMini is just the HUB of that wheel.
Multiple 4K monitors, possible upgrade to an external GPU and even better monitors is purpose driven.
OS, Application hosting, and optional scratch-disk use in the SSD is do-able... but archival storage should be on a desktop RAID at a minimum, anyway, and off-site backed up preferable, with business-class multi-Terabyte redundant, hot-swappable storage, if not a thunderbolt-connected NAS or 10Gig-Ethernet connected SERVER-hosted SAN, if it really becomes an issue in a professional production environment. On-computer storage is more of a problem than a solution in this day and age.
Arguably, the next step beyond that becomes network access to virtual machines, rendering farms, cloud servers, and other infrastructure-as-a-service computing... where even the processing is off-loaded into a server rack in an environmentally controlled and secured location, and doesn't happen directly on the workstation as much. (sure there are still examples of isolated field work where local processing is important), but there again, the MacMini becomes an interface window on a larger computing world, not a computing sphere unto itself... and theoretically doesn't need to have the heat, power draw, or largely un-tapped reserve processing power on every workstation desk.
Combining the video needs, and off-loaded storage needs and higher-level processing power functions, with other thunderbolt peripherals, and USB-connected interface devices creates a workspace environment where components can be changed out, which is where iMac doesn't quite suit as well, with fewer ports, and an integrated screen that doesn't match with a pair, triple, or quad monitor workstation.