Pretty much agree. Apple may not be the only manufacturer building fully soldered RAM and storage, but they're amongst a tiny minority, and as much as iMacs were RAM upgradable, RAM was the least of the problem.
I can't speak to what the Mac mini was designed for or why the product seemed to lurch along as it did. I tend to assume Apple were simply investing in it when they felt they needed to, and ignoring it otherwise since it was ostensibly a low-profit model. It was clearly designed at first at least to be sealed and only opened with specialist tools, not for users to upgrade. Indeed, from Apple's perspective it appeared they rather anticipated that users would simply toss them away if and when they needed something better.
To my mind, speaking as someone who has worked in IT for years, I enjoy messing around inside computers, but I also know most people don't, and that Apple's core belief in appliance computing is not only more profitable for them, but also more aligned with the majority of customers.
It's certainly true that outside the Apple product range, computers are invariably more open in architecture and build, and there is certainly nothing wrong with that approach. Even my mini PC, which is smaller than a Mac mini and far cheaper, has upgradable RAM and not only a replaceable SSD, but also a slot for additional storage too. I open it up and I m so tempted to mess! But it reminds me of something I said to a colleague a while back, that I use Windows every day because I manage Windows systems and networks, but I go home and boot up my Mac, because that's for getting stuff done. That's how I see the direction Apple's system policies have gone and will likely keep going - a computer you buy and use, not one to mess with.