I see it this way.
Firstly, Apple in terms of its core customer has zero competition. macOS forces you in one direction, Apple devices. Sure Hackintosh but that is a small enthusiast community. If Windows was as good an alternative you are already using it, not macOS.
Secondly, because of the first part, they can control the pace that devices are upgraded far easier, soldered ram, CPU and so on. Offering a pc style system where a core customer swaps out the CPU, Ram or GPU themselves removes that control. Potentially they could keep that system for the next 10 years. That is no use whatsoever to Apple and they won't risk core customers doing that.
Of course, some will keep an existing system for that long anyway and to apple's credit, they ensure support for that long. But that person is not core. Core customers are those that want to flash the bling as soon as it's released.
I don't see it as common sense but rather wishful thinking on your part. I recall people saying the same thing about the Mini.
Outback XT was a great car, loved mine.
Do either of you have any evidence to backup that there are a large number of users interested in swapping GPUs, CPUs, etc? My evidence is what I believe to be common knowledge that people do not do that, and those that do makeup a minuscule percentage of the buying public. Apple sold almost 20,000,000 computers in 2017. How many people do you honestly think want this tower or to upgrade things like GPUs and CPUs?
If I look at reddit, r/buildapc has 1.3 million subscribers. Even if Apple enthusiasts who fit your description equal that number (they don't even com close, but let's say they do for fun), you are still at most looking at around 6ish% of the number of computers sold last year by Apple. If we admit to ourselves that those users wouldn't buy every year, and more like every 3-5 years, then you are looking at an even smaller percentage. Even if there were a million users that fit your description, that would not be large enough for Apple to justify making the machine you and I want. And the fact is, the number is probably closer to 1/10th or 1/20th of that when you factor in how many fewer Mac users there are to PC.
Also, one last bit for the record. Most Windows PCs are not truly upgradeable either. Sure their components are slotted, but that does not mean they are easily upgraded. Most prebuilt PCs have proprietary power supplies that are specced at the minimum requirements for the system, and upgrading to parts that require more power can become very cumbersome. Further, they often do not use off the shelf motherboards that offer less expandability or less future proofing, and most motherboards only last for a couple of chip generations before a new socket or chipset is required. So while it is possible, it is not as straightforward or pain free as many describe. Most PC enthusiasts that like to do that kind of thing build their own or buy from small shops that use off the shelf parts for this reason.