Unlike with the @1x -> @2x transition we had in 2010, this time we’d only have a 50% enlargement rather than 100%. The thing is, a 50% enlargement with interpolation doesn’t look worse than a 100% enlargement with pixel-doubling, despite the loss of details due to the interpolation...
As you can see, older non-@3x-optimized apps would actually look better on an @3x iPhone than non-@2x-opitmized apps did on an @2x iPhone. Add to this the fact that the screen's pixel density would be higher this time around, and the perceived image quality difference would be even smaller.
Take a look at the images below to see the 50% upscaling visualized. For more iPhone 6 updates,