I'd done countless tests, and a pixel doubled image looks 502327% better than the upscaled blur of crap retina does.
Are you talking about view contents like photos/videos or about UI bitmap elements like icons or buttons? Because Preview, QuickTime and other apps do exact pixel quadrupling of content on a retina display. Even most games do. Funnily, this isn't possible on the PC. On Windows, there is no way to exactly pixel-quadruple a game at 1080p playing on a 4k monitor, and gamers have been complaining about it for years.
It's true that the non-retina apps get blurry, but it's not the "nearest neighbour" algorithm, since it would mean pixel doubling. It's a more complex smoothing algorithm. I'm not sure why appkit doesn't just quadruple the pixels in that case, I suspect it's done on purpose to single-out non-retina apps/widgets.
But the system does exactly quadruple pixels on App content. It's easy to see on screenshots.