The more I think about it, one last refresh in March makes sense, especially since it's been so long and it's bad for marketing to show that they're increasingly slowing their refresh rates. Two updates in 2016 is technically better than one big one.
Siri is one thing; not everyone likes /uses / wants Siri. But it's clear Apple knows Siri is the future and is pushing it very heavily across the product line. OS X adoption was inevitable.
Touch ID is another thing. I think more users want this when compared to Siri. Apple values security incredibly highly, especially given recent events. MacBooks are still in the mobile category, like iPhones and iPads. As Apple continues adopting Touch ID for things like Apple Pay, Notes, etc. it doesn't make sense to leave this feature out of newly designed mobile machines. More and more websites, banks, credit card companies, etc. are adopting Touch ID on iOS apps. Why can I not utilize the same level of security on a MacBook?
Apple's goal with the rMB was to make the most forward-thinking, advanced laptop that they could, without disrupting existing supply chains for the MBA, and MBP. It was a design-driven, from-the-ground-up machine that had nothing holding it back. A design-focused product meant compromises for the user, like a single, new port, a wonky keyboard, a 480P camera, a mobile processor, etc. They knew offering an incredibly thin, light, and fan-less machine wouldn't be cheap. If it was a MBA replacement, why wouldn't they brand it as "The New MacBook Air" or "Retina MacBook Air?" The rMB is its own product parallel to the MBP and MBA offerings. It's more similar to an iPad than a MacBook.
I'm not sure why everyone is so quick to throw the MBA to the curb, when it's a significantly different device. It offers flexibility, usability, and an attractive, low-cost-of-entry price into the OS X environment. The MBA is for the students, the casual users, the parents and baby boomers, the non tech savvy, etc. These people have SD cards and USB-A devices. They don't care if their machine has a fan, nor if it has a retina screen. All they want is a Mac laptop without breaking the bank. I'm also not sure why we're so quick to want to reduce the MacBook line back to two device categories so quickly when almost every other Apple category has three offerings.
I see something like this playing out:
March refresh of MBA, MPB, MB with Skylake processors, upgrade to TB3 over USB-C, possibly other minor things like ForceTouch on MBA. Possibly reducing MB price, and reducing costs of SSD upgrades across the board. A guts-focused snack to settle our immense hunger for a bit longer. Then in the fall, we'd get the MB design language adopted to the MBP and MBA (MBA might get some, not all features), with brand new unibodies, fully redesigned and smaller logic boards, new keyboards, new battery technology, color options, etc. Then all three laptops would gain hardware for Touch ID, Siri, possibly even Apple Pencil support on the track pads.... more radical things that they'd need OS X 10.12 for. They'd package it just like iPhones: two products in one, hardware and software in perfect harmony.
