He believes the cost, thickness, and production concerns will continue to keep Retina displays out of Apple's lowest-cost and thinnest notebook line for the time being.
Retina MacBook Airs might not happen until they use two specific technologies:
1. More power-efficient display technology (OLED, for example).
2. ARM-based CPU: lower cost, lower power consumption.
Yes, an AX SoC would require recompiling OS X and all apps. But don't forget that OS X was originally developed on RISC-based PowerPC CPUs. It might be as simple as selecting "Universal ARM / Intel binary" in your Mac app's Xcode build settings. Apple has transitioned their user base and developers from 68K to RISC to Intel already. Been there, done that. And they could easily do it again.
The arguments against ARM-based MacBook Pro break down into:
"Not 64-bit"
The ARMv8 spec, which was released in late 2011, includes a 64-bit instruction set.
Just a matter of time before Apple ships 64-bit AX SoCs.
"Not powerful enough"
Apple could easily develop quad-core AX chips.
Also just a matter of time.
"Tim Cook said 'no'"
No, he didn't. CNET's report implies that converging iOS and OS X (and their respective devices) will never happen. But Cook never said that the MacBook Air hardware roadmap will never include AX SoCs:
Apple feels iPad satisfies--or will soon satisfy--the needs of those who might have been interested in such a product.
There will always be a segment of the mobile computing population that needs to touch-type on a physical keyboard (students writing lengthy papers, journalists / bloggers, etc.). And there will always be a market for mobile computing devices with screens bigger than 9.7" (designers, programmers, engineers, photographers, etc.) So the iPad will never totally replace the legacy "laptop." Think back to Steve Jobs' trucks vs. cars analogy.
MacRumors story:
https://www.macrumors.com/2012/02/04/arm-based-macbook-air-unlikely-based-on-analyst-talk-with-tim-cook/
But a physical keyboard doesn't require a smoking-hot Intel CPU to keep up with your keystrokes.
Does it?