Coming from a 17" MBP and still using it... I disagree. Nothing (to me) beats the workspace of a 1920x1200 canvas. At Retina @2x resolution even more so.There's always a compromise. The current 15" retina screens have 2880x1800. That corresponds to 1440x900. But earlier 15" non-retina MBPs had 1680x1050. So Apple could make a 3360x2100 screen. That's still around 85% of a full 4K resolution, as compared to 63% now. Full 4K would make the system elements (title bar etc) too small to use anyway, when used in a 2x retina mode on a 15".
My only problem is... I tried the scaling option with the rMBPs and the new TBMBPs... it's pretty bad. Generally text and stuff still looks sharp enough. But once you start working with any design app and have to draw lines or similar... the scaling is killing it. Lines start to flicker as points no longer correspond to pixels.
Edit:
Supporting what I said with a longer explanation and a nice little animation:
https://bjango.com/articles/macexternaldisplays/