A large number (most?) of people who buy a computer are never going to repair or upgrade it. Of those machines that are opened up, a large number will be repaired or upgraded by a technician or support staff and even then, the vast majority are only going to have RAM increased. Gamers and people with a technical bent like to be able to access their machines, but for almost everyone else, they are appliances and accessibility just adds space and cost.
I'd also love to see something between the mini and the Pro in a tech-friendly format, but both of us belong to a very small market sector and one that Apple has no interest in.
Macs used to remain useful for longer than Windows PCs. Part of that was OSX, part of it was different users and use cases. AIOs made no sense in the PC world, where you'd need to be throwing the whole thing away every couple of years, rather than upgrading components. It was a better proposition on the Mac side of things. These days, however, a 5 year old mid-level PC is still useful. A Windows/PC AIO isn't as bad a choice as once it was. Microsoft can make decent hardware, but suffers from coming late to markets with products that are as good as _but_no_better_ than those already in the field. If it can avoid playing catch-up with Apple and the iMac a Microsoft AIO isn't necessarily a bad concept.
Lot's of 'ifs'.