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.