I visited the Apple Store (Singapore, Orchard) today with no success. A specialist and his senior colleague said that this is out of warranty and there is no repairing program for this issue. However, one of them mentioned that last week he repaired a similar problem for another iMac. The senior specialist was very friendly and spent almost an hour with me but couldn't suggest any solution. He insisted Apple doesn't make any exceptions and free repairing is impossible. My arguments that many people managed to get their iMacs fixed for free didn't help.
He also mentioned there are no any more senior people in the entire Apple Store so there is nobody to contact and complain. The price for repairing (full screen replacement) is more than 800 SGD (600+ USD) + GST, etc.
P.S. I will never ever buy iMac again. Problems happen to any devices but only in Apple repairing costs like a new computer.