A key factor in accessibility/upgradability is manufacturing cost. That lovely Mac Pro chassis costs $$. Each access panel, card slot and drive bay increase parts count and cost, each access screw an additional assembly cost. The ability to support add-on gear requires a larger (and hotter) power supply...
A company with a profit margin of more than 30% could easily afford to pay a chinese worker 50ct for adding some additional screws. At least the other PC manufacturers have no problem with that.
Does anyone buy a car anymore because it's easy to repair? No. Not even Consumer Reports notes whether it'll cost more to fix because of the way it's put together. And the ability to hot-rod it? Such modifications are reserved for vehicles built in the days before emission controls and CPUs under the hood.
In a PCs lifetime it's very likely that the harddisk will break or that the RAM will fail. What's your response if your 27" iMacs HDD breaks after 3 years ? Buy a new one ? It's a desktop computer, not a disposable mobile device that you can easily replace every two years.
A car's engine usually makes it until the very end.
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External Thunderbolt.
So a small hatch to access RAM and HDD totally breaks the aesthetics of the iMacs design, while external boxes for boot disks are ok ? Isn't that a little bit contradicting the idea of an AIO ?
It seems that some people make a PC tower from the iMac, just without the case.