Earlier quoted context omitted.
No one actually cared that a laptop needed a fan, though. Apple could have put a fan in this and no one would have seen it as a huge deficiency.
Still it's massively useful. It means more space for the battery and it also indirectly means that if you need no fan, you probably generate very little heat, which means you have a crazy efficient CPU meaning even more battery. Which is how they can do the same amount of battery life as the 11', despite having a bigger screen and a retina display. And a big part of that equation is probably the fanless chip. But it'…
This coincides with being fanless.
Had Apple made this (say) the same thickness as the MacBook air, they'd have increased their volume by a measurable amount, they'd have been able to have a much more powerful processor, and no one would be able to say that ARM is a good proposition for them for the foreseeable future.
Now they have a fairly weakly performing computer, with an ARM-sized logic board, which is going to be bought by people who don't care about bottom line performance.
This is going to be the slowest Mac in a number of years - and if it's a success then making the next-slowest Mac be ARM wouldn't be such a jump.