People don't realize that all-in-one, integrated, non-repairable electronics are, in the long run, cheaper., and the majority of consumers are cheapskates. Cheaper electronics always wins out in the long-term, and will largely drive out modular, more repairable products, regardless of superior upgradeability or maintainability.
You can't compare computers with cars, as your typical consumer computer doesn't cost anywhere near a car. And, no, the retina MacBook isn't applicable here, because it's bleeding-edge, first-generation technology, which is always expensive (plus, there's the "Apple tax"). Just wait until the technology matures and the costs dive.
Also, if anything, the move to tablet-based computing is going to accelerate the drive for lower-cost (integrated/non-repairable) desktops and laptops.
Anyone remember when TVs were actually repairable (discrete transistors, anyone)? Anyone remember Sam's Photofacts? Yeah, thought so.