Definitely on the MacBook -- there's no way to outpace the entry-level (price-wise) Mac notebook. Against the Pros, who knows.Also, I'd have to guess that the MacBook and MacBook Pro sales (*individually*) will surpass the MacBook Air's once it's "newness" wears off.
But with Apple's history of keeping niche products around despite slow sales (hello Mac mini and iPod shuffle) and giving products a significant tweak instead of throwing in the towel (AppleTV Take 2), I think you're being a little simplistic to think that the Air's going bye-bye just because it won't sell as many as the MB and MBP.