that is exactly in apples interest if people upgrade at very least every 3 years , the problem is if the smokey bit and the cost get public who will pay for a used iMac with such a problem ?
i mean after 3 years your iMac will be worth how much? if its the i7 ? in theory 1300-1400 ,but if potential buyers know a new display cost them 700 ,they would need to pay more for the used one then for a brandnew one so the resell value will quickly drop to 500 or so if people who buy used Mac's have a bit common sense left as its a reoccurring problem so even the naive will think about what if that bit apple care left runs out ...
all these little problems that occur on the new iMac's have a impact on resell value , so the whole financing of a new iMac by selling the old will collapse, just a thought as i think no matter how good the iMac looks but paying serious money for something used that likely to end more often in the store to get repaired is not a great idea to buy