i would say that a substantial majority of people who purchase the rMBP could actually get away with a $900 1080 windows laptop (or roughly a $1700 Macbook Pro). that's from a spec standpoint. if you are buying the rMBP you also value portability and form, thus one must consider the physical attributes as "features". this is something that the industry (with exception to Apple) is still getting used to. sony has tried for many years to present physical attributes as features. while they were successful early on with their very portable VAIO laptops, it didn't stick with consumers since the windows experience with XP and then Vista was far from polished and thus degraded the inherent value of the physical attributes. computer manufacturers (and technology manufacturers) have historically always correlated price with specs. it took Apple to break that correlation and introduce physical attributes that while might not seem like features, are attributes that come into contact with the consumer more than a spec bump.
the fact that the rMBP and the MBA cannot be upgraded is countered by the strong resale value on craigslist, ebay, etc. looking at the MBA, consumers are finding that through resale values that they can buy into the latest gen for only an additional $300-$500 (depending on configuration, which only helps the consumer as time goes on and the standard of memory/storage becomes cheaper). thinking about that from a windows laptop standpoint, that's like buying a regular laptop at staples every 2-3 years. so that overpriced Apple product only takes a chunk out of the consumer's wallet when they first buy the Apple product. once you clear that $2,000 hurdle, resale carries you for as long as you want. this is the same reasoning why people pay extra for Subarus. a comparable Impreza (to the Corolla/Civic) will carry a $2,000 price premium easily. however when the Impreza is ready to be traded in, it will hold its value by nearly that much extra