Everyone has a theory and prediction about MacBook Air retina or the lack of it. While some use technology argument I used customer value perception argument to explain why Apple did not bother to offer you one in the last upgrade.
Here is what I wrote then
Apple seems to be able to capture almost all value it can from MacBook Air with its current pricing and by positioning it as better portable with longer battery life. There is not much value gap left between the two models that Retina display can fill.
At that time the product price points – value map showed no space for a MacBook Air retina,
Apple chose to not give you retina display on Air not because of technology difficulties or cost reasons but because they have not yet figured out a way to extract their share of the value in the form of price premium. The first step towards that value capture is value signaling and it is happening with the recent realignment of MacBook Pro models and pricing.
Apple eliminated 15″ non-retina, spinning disk (hard disk) MacBook Pro models but kept its 13″ brethren. The only logical reason for its existence is as a seat warmer for impending 13″ retina display MacBook Air.