uhhh... the iPad 3 and 4 are thicker, heavier, take longer to charge (bigger battery), and has worse battery life than the iPad 2..... Apple cannot defy the laws of physics (no matter what their ads convince you).
Sure maybe Apple could create a rMBA that achieves the same battery life (due to improving battery tech and power reduction on haswell), but that only means they could keep the same screen and improve battery life by an hour or two. Plus the rMBP have some performace/graphical degradation because the processor has to work so hard to push that many pixels. Tradeoff isn't worth in IMO for an entry level, consumer device. Especially one that prides itself on being thin and light.
Minimally thicker and heavier - sure they take longer to charge, but that wasn't what you said. You said you didn't want to sacrifice battery life.
Across the board for Apple products, the battery life either stays the same, or very slightly decreases when a retina display is added.....
Plus Apple isn't in the same place now as they were when they made the jump from the 2 to the 3. The next iPad is supposedly going to be lighter and thinner than even the 2.....
The rMBP gets the same battery life as the cMBP AND is lighter and thinner. AND was released more recently, giving you a better indication of what Apple is doing/can do with the battery tech they are using.
But relax, they've kept the cMBP around as a cheaper alternative, I'm sure they'll do the same for the MBA - at least to begin with. So you have nothing to worry about!