Apple designs products for the masses. I don't think people who want to hook up 3 external displays to a Mac mini represent the majority.
I think the last couple of years - since the iMac Pro, continuing to the revamped Mac mini, the announced new Mac Pro, and now the new MacBook Pro - Apple has returned to actually offering machines that truly deliver what "pros" want.
My point wasn't so much that "the average person" wants to hook up three displays to a Mac mini, my point was that while the TB3 ports can do - as you said - practically anything, the HDMI port on a Mac mini not only can't do anything else, it
prevents use of a third DisplayPort display.
I'd wager a similar scenario would play out on a MacBook Pro with a HDMI port - it would reduce the total number of DisplayPort outputs by one, to provide a dedicated display output to the HMDI port, which the user may or may not want to use.
I agree it's possible to argue that the Mac mini is not targeted at "pro" users. I
don't agree with that argument (partly due to how usable it is as a workstation, and partly because they literally talked about it's use for "pro" use-cases at launch), and I use one as my daily workstation, but I can see how the argument can be made. It's their cheapest desktop, and literally is designed to allow "BYO" for KB/Mouse/Display. The HDMI vs DP issue on a Mac mini is kind of moot anyway - it's going to have other issues related to running three displays before "HDMI vs DisplayPort" is the biggest thing wrong, but that wasn't my point, remember.
The MacBook Pro is
absolutely aimed at "pro" users. Do some people buy it just to browse facebook and MacRumors? Probably. If they want to spend $3K to do something they could also do on on a $300 iPad that's their choice, but it's absolutely targeted as a "pro" machine. This is why I find it so ridiculous that so many people claim "oh but pros want SD cards".
REALLY? They don't want XQD/CFExpress card slots? They're happy with SD card slots hobbled to USB2 bus speeds?
I find that quite hard to believe.
To expand on the 'suit the masses' counter argument: if Apple were making the MBP to "suit the masses" the way other companies do (aka trying to be jack of all trades) it would do what every other PC out there does - it would have **** like 2xUSB3.0, 2xUSB2.0 (yes, really, in 2019), 1x USB-C (possibly 3.1g1, possibly 3.1g2, maybe TB3 if you're lucky), a ****ing HDMI or mini HDMI or some other ******** port, etc. Notice what's missing? Lots of high-speed multi-use ports.
Go look at a "high end" Dell or HP or whatever laptop. How many of them have even two TB3 ports?