I think Apple also realised that the 2x solution is all they will need. There is very little point going higher density than the current 15" rMBP display. So there is little point in supporting completely arbitrary resolutions — Apple are betting on this density as the standard for the future.

I think they also realise that our current "2x" solution is our future software's "1x" scale factor. When Apple's line of hardware is retina-only, we'll probably see them drop support for the distinction between 1x and 2x. Everything will simply become designed for the current density.