They won't do it because they have taken Steve Jobs' vision of end to end control to an extreme level. Jobs was apparently still in favour of some expandability, if not the nMP would have come to market sooner (and the Cube would not have been killed). As soon as he died look what happened...
Mac Mini went from quad core with two upgradable SSDs - to dual core with no upgradability
Upgradable workstation Mac Pro turned into the nMP
MacBook Pro lost the upgradable SSD, upgradable memory and replaceable battery. Jobs made a big deal about the user upgradable SSD in one of his last keynotes.
The operating system has become more walled off, the APIs aren't being kept up to date, the Finder is crusty, the network stack is flaky, it doesn't install on Macs that can easily run it. This was Jobs' baby. He was proud to call it the most advanced OS in the world. Now Linux is more advanced in many ways. Windows is far ahead.
The Mac Mini went the way it did not because Apple is like evil. Or, anti-upgradeability. I mean it's unfortunate that the things that customers wanted were taken away like upgradeable RAM and HDD's. But, to me, the Mac Mini is totally align to Apple's vision. And, this vision, BTW, is not just external aesthetics like how the Mac Mini looks from the outside. But, also internal design, or how the Mac Mini looks from the inside. I think the current Mac Mini lacks those upgrade paths because Apple wanted to make the inside more "elegant." This is just my opinion. But, I think that's why it is like that. And, Apple thought that with the Market the Mac Mini is intended for that it is totally fine.
I mean, honestly, if you look at used Macs online, like Craig's List, or Ebay from original owners, you will find that the Macs on sale are basically left stock. Like, they never bothered to upgrade them after all these years.
My point with that is that Apple does "care" as much as you like to say they don't about their "target" audience. In fact, they "care" too much, I think, that the unintended target costumers of Mac Mini's are left wanting more because Apple did not look at a Windows Manufacturer's website with the same form-factor as the Mac Mini, and say, We gotta make our Mac Mini like that. Apple made the Mac Mini according to a certain vision. A vision that you don't agree with. And, I don't agree with, either. But, I see where they're coming from.
We can all blame everything on Steve Job's death. But, I don't think there is any legitimacy or reason to think that there are forces within Apple that relished or used the absence of Steve Jobs to push a certain vision contrary to what Steve would have liked or wanted.
It doesn't work that way.
I don't know what to say about your qualms with MacOS, though. I like the Finder. It's whatever. API's? I'm not too familiar with that. Network stacks? Not familiar with that either.
Nah, man. I have a Windows 10 PC for gaming. I don't think Windows is more advance. It's still Windows underneath it. I mean, have you tried troubleshooting Windows? OMG! But, for gaming, Windows beats Macs by a landslide, if you're into Triple-A games.
But, if you console game and have a Mac, I think you're fine. Trust me. I already spent a certain amount of money towards my Windows gaming PC by getting a 144Hz monitor, mechanical KB, nice gaming mouse, etc. So, I'll be using it for a while. But, have you tried updating Windows 10 to the recent Windows Anniversary Edition? OMG! Nvidia driver updates that have been sucky as of late breaking basic Windows function? OMG!
Linux? I used Linux briefly. But, it was having issues on my iMac not running the fans correctly. So, I uninstalled it. I would like to revisit Linux again, though.