3K is nowhere near the point of no return on a 27" computer display. The old 27" iMac is nearly 3K at 2560x1440. 4K on a 27" screen might be reasonably "retina", but the scaling would be all off in OS X, among other potential issues. It would also be a bit on the low end for being truly "Retina", and wouldn't look retina-sharp if you scaled the resolution at all. On the Retina MBP screens being able to scale the resolution and still have it look incredibly sharp has been a great feature and I'm sure they want to bring that same flexibility in screen real estate and scale to the retina iMac as well. With the way they do doubling resolution to make the retina displays, 5K makes perfect sense. And it's not so far overkill on pixels that it's just specs for specs sake (the way that say 2560x1440 on a 5-6" phone screen is).