I absolutely assure that the entirety of OS X has undergone a major redesign with Yosemite. iOS 7 was a redesign of the workings and motifs used in the mobile operating system, and OS X followed soon after.
Here's what you're saying from a designer's point of view. Early humans were a pretty good set of species, already advanced in terms of intelligence and such. But over the years they gradually evolved through small changes in their genetic code, tweaks set in motion by what works best. Eventually, those tiny, seemingly minor "under the hood" updates, changes to bone structure, and body hair distribution led to a radically distinct species known as homo sapien. This species was better in nearly every way, from having a larger cranium capable of containing a larger brain, to being taller, faster, more agile, and better suited for creativity and technological advances...but obviously that doesn't matter AT ALL because the way this species came to be was through minor, seemingly useless evolutionary quirks. You get me?
Also...
Windows 3.x to 95 was an evolution, not a redesign. They shared extremely similar, if not the same design, just with improved underlying workings. As for Windows 7 and 8, that evolution was most definitely not a redesign. It was an addition to 7. 8 still functioned just as 7 did- it simply had a new app launcher. The skeuomorphism was almost entirely thrown out the door and the launcher was optimized for touch devices, but other than that, literally nothing else changed. You can't slap a new launcher on an old OS, oversaturate it, flatten it, and take away an alternative option to said launcher and call that a redesign. Particularly if it's not even for the entire OS. The launcher was a redesign, Windows 8 itself was not.
In conclusion...
Just because something is small and iterative doesn't mean it's not important. Design is all about tweaking, improving, and evolving the interface and inner workings of an OS. If every year an OS doesn't scream "Hey! Look at me! I've completely dispensed with all reason and pursued a completely different direction, changed everything from the ground up, even though we just did that with our last few iterations," doesn't mean it's not a redesign. It doesn't mean it's not important, or worth getting excited about. Actually, it's the opposite. It means OS X gets better, faster, more efficient, and more empathetic. That's arguably just as, if not more important than big, revolutionary announcements (which, if you pay attention, OS X has had for every version in the recent past).