While it is true, that Apple has announced the Macbook Air 4 years ago, they also did update the Macbook Air several times already. The last version, that is already selling more than half year ago has the same i5 and i7 you talk about and besides, it has much advanced technology - Thunderbolt.

Many people do not really understand why Thunderbolt is so advanced, especially more ultra portable notebooks. In essence it provides extension of the processor's PCI-Express interfaces outside the computer. In the Macbook Air of 2011, you have two 10 Gigabit/second channels out of that tiny connector. You can connect almost anything there, that is traditionally found inside a conventional "PC". No need to make your notebook bulkier or more power consuming (each and every additional chip consumes energy and takes pecious space).
Apple already provide quite useful 27" display with built-in Thunderbolt expander etc.

In the end, it all comes to competition. No doubt Apple will soon release a new update to the Macbook Air, that will bring about new performance levels... what then?