Retina is the eventual destination for all displays in all lines. The question is if the transition will take weeks or years.
About 1.5 years ago, many naysayers here on MacRumors claimed that Apple would not consider a retina iMac 21" any time soon. They said that the low-end iMac was not a priority for retina enhancement. They said it would make the model too expensive. They said it would require so much better GPU etc etc. Well, a retina 21" iMac was released a couple months ago. Sometimes, it's just a matter of when, not a matter of "will they ever?".