The thing is that the 13 Air and the 13 pro are getting very similar. If they get any closer in portability there will be no reason for the air.
The only way you can make the 13" rMBP more portable is to remove features that rMBP buyers want. The only way you add features to the 13" MBA to bring its specs in line with the rMBP is to compromise the portability that MBA buyers want. I just don't see it. On paper the specs seem very close, but holding them in your hands they're a chasm apart. And that's as much a function of design goals as it is technical limitations.
And, failing that, even if there is the possibility of convergence for the 13" MBA and rMBP at some point between now and 2016, you have to completely ignore all the other portable offerings from 11" MBA up to 15" rMBP to draw the (imho flawed) conclusion that Apple is working towards unification. Heck, I don't think it's out of the question that we'll see the return of a 17" MBP offering as soon as the technology allows Apple to build a 17" rMBP realistically. I think they dropped the 17" because GPU/battery/panel cost made a 17" rMBP impractical and they didn't want to have their "flagship" largest laptop not be Retina when the 15" was.
You just can't make a MBP as svelte as a MBA without removing the "P" from what it is. You will have had to ditch user-upgradable RAM, storage flexibility, and external connectivity options. And, similarly, if you stuff a lot more cabaility into a MBA, then you've stepped far away from the portability profile that attracts MBA buyers. Some other vendor will be happy to make a smaller, lighter, more battery efficient product that doesn't have burden of trying to satisfy both markets.
It just doesn't make any sense to me. Apple would just be left with a compromised middle-ground product that doesn't satisfy its current market. There's room and need for both "airs" and "pros" in Apple's product line and the steady march of technical progress doesn't seem like it will change that reality any time soon.
MBA and MBP serve two different markets. Ignoring that doesn't help anyone.
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