The thing about the past, especially in regards to computers, is that regular updates were necessary given the pace of improvements made in so many different areas. And back then, Apple was still considered to be moving at a snail's pace compared to most PC manufacturers who would launch a new desktop or laptop and you'd be reading about the newer version replacing it in months, or less. They might leapfrog ahead of everyone with some computer that is far more powerful than anything else out there, but that never lasted long.
The past couple of years, improvements and innovation in the computer industry have been dictated by the annual (if we're lucky) updates to processors. And what you notice with the new tech, if you notice at all, is that it's more efficient but not necessarily faster. And given Apple's products tend to last 3-5+ years with little if any change in physical design, a lot of people just don't see the need to update annually. I say that as I work on an early 2013 15" rMBP that still does everything I need it to. Heck, I still have a 2003 G5 tower I bought new that's still running (achingly slow), which I still think is one of the most perfectly built computers ever.
Frankly speaking, I believe that if Apple were updating all their machines annually, by simply tossing in the latest chips, it would negatively effect the value (perceived and real) of their products.