Averaging over the course of OS X's existence doesn't really make sense. Conditions have differed drastically throughout. Frankly when OS X launched it wasn't great. I would argue that Jaguar was the first truly usable version. Hence the hurry to refine it in the early years. Once they hit their stride with Panther, Apple shifted to a 2-year model. That was the plan for quite some time. Then, thanks to iOS's influence, last year they moved to a one-year model. Mavericks is a slight aberration due to the perceived need to greatly overhaul iOS's design language. Though it'll still arrive this year, obviously.
As far as why Apple's changed from 2 years to 1, I'd say it's from a desire to differentiate themselves from their competitors, and to keep the Mac fresh, and try to encourage more growth for the platform/stem the overall decline of the desktop PC marketplace. Being able to talk about year over year new features is a lot more engaging for users than just doing it once every two years.
The key difference has been in adding truly features to the OS over those releases, versus simply catching up.
I mean when OS X was really adding functionality, allowing new things to be done or in easier ways versus just supporting new hardware technologies; or just making it compatible with Intel processors; or just simply catching up or fixing annoying things that didn't work well the first time they were tried, like cloud services before they were even called that.
The competition for iOS was bleak at the beginning, and then some of them hit the market like a train at full steam; and this next iOS iteration seems to be a mix of what makes sense to catching up.
More hardware features will drive better OS, and real game changers in Software Apps will also help to make better OS.
The mouse became the trackpad, and the keyboard became...still the keyboard...some attempts to make it the touch screen or some dictation thing...but it isn't there yet...we are still faster making some keystrokes...imagine writing code in a different way...
If Apple pursues buying or implementing something better than Lytro cameras, it could drive some more things for the OS.
3D printing will be something else that can make a difference, the OS will evolve as features are added.
When specific apps or hardware gadgets become the natural way of doing something with a PC if when they make sense to become part of the OS, when they make sense to Apple...sadly not necessarily when they make sense to us (MacRumors enthusiasts).