It's happened before. 15" rMBP introduced before 13" rMBP, riPad introduced before riPad Mini. Is it possible that Apple will introduce a 21.5" riMac? Maybe not 5k, but maybe with a 2560 x 1440 display to increase pixel density?? I like the idea of a retina display desktop, but I like the size of the 21.5" screen. Thoughts??