You are wrong.
First, pre-2012, Apple encouraged users to
service their own memory, install or replace
AirPort / WiFi cards, and replace their
own hard drives, oh, and
replace their own batteries (back when they designed these things to be replaced).
It is only since approximately 2012 when Apple started
actively designing against repairability and user-serviceability that they have promoted this idea. Until then they actually designed features into their machines to make them easier to service, like these
cool memory latches in the plastic MacBooks.
Second, Linus himself worked for many years at NCIX in a capacity higher than a "basic technician", and from what I've seen of some of the rest of their team, they certainly have enough skill to perform maintenance on an iMac. The only reason why it's difficult to do this is because Apple uses techniques that simultaneously save
them money (not you), and
increase the knowledge and tooling (not necessarily difficulty) required to repair their machines, and then do not publish service manuals.
Third, "shoddy third party cables" do not
frequently lead to permanent hardware malfunctions. In fact, only a few cables (Thunderbolt, USB3, and FireWire come to mine) allow voltages high enough to really damage hardware, and in Apple's defence (for once), they are one of the best in terms of adding protection circuitry to their logic boards to prevent poor quality cables from damaging the hardware.
Finally, who do you think makes the internal components of an iMac? The logic board is manufactured by a third party such as Foxconn, Quanta, or Pegatron. The memory and SSD chips are made by a third party such as Micron or Samsung. The display panel is made by LG or Samsung. The power supply is again manufactured by a third party such as AcBel.
For single-machine nonstandard components such as logic boards, there are usually no aftermarket manufacturers. For things like RAM, SSDs, spinning HDDs, there are plenty of manufacturers such as Micron, Samsung, Kingston, Corsair etc al that make products that are
better than those Apple include with their machines. This is just one of the reasons why Apple abandoning the standardised parts they used since moving to Intel over the last few years is such a hostile move to consumers.
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I can certainly see where Linus comes from when in his latest WAN show video he discusses the cost of the parts required vs the cost of the machine.
The cost of these parts
to Apple are going to be
far less than the retail value of the machine. I'd expect the display part to cost perhaps 10-15% of the retail value of the machine, the logic board another 5-10%, and the power supply 5% or less.
However, Apple has a history of massively overcharging customers for spare parts such as logic boards, so it is entirely possible that they would charge $4000 USD for the part required to replace the display alone...