I think that is the real question, "Why do people still buy ..." and it speaks to that percentage of Apple's base that want a machine with an optical drive. So for that segment they will pay for the drive and sub-par display performance.
If you look at Apple's product releases over the years, and with overlapping products, you can really see Apple market research at work. iPad Pro 9.7 vs iPad Pro vs iPad 9.7. How many people want pro/not-pro, how many people want pencil/not-pencil, how many people want big/little. One of the Apple product managers said "I survey customers by counting how much money they will give me for a feature." His point was that customer surveys of people who aren't buying are aspirational, but what people actually pay for, that is actionable.
The question is no doubt being pondered, "If we offered a retina MBP with a built in optical drive, how many people might buy it?" The tooling of the body cavity its clearly an investment that you don't make for less than a million or so machines.
But I agree that the fundamental question is flawed, there is no reason to "upgrade" a product that is meeting all of its customer requirements. But customers asking the question suggests that requirements may have shifted.