Lets be honest here, Lion, Mountain Lion & Mavericks are just small incremental updates. OS X clearly has not been a priority hence why each 'new' OS since Snow Leopard has been nothing more than a bunch of improvements & a scattering of new features.
Whoah there. A huuuge list of changes go into every major version of OSX. It's just that the bulk of those changes are not immediately user-visible, but changes in APIs, libraries, drivers, etc. Most of the time Apple chooses not to market those changes because most of its audience has no idea what any of them entail. They just care about stuff like 'does the Chess program have new graphics'.
I honestly believe they are busy working on a new operating system that somehow will be a fusion of OS X & iOS and will work across all Apple devices, be it Macs or 'i' devices. I don't even think it will be called OS 11, it will probably be called something entirely different.
Having just one OS to maintain & support just makes sense. I would hope that it will be adaptable and more feature-rich for Macs, but that's for Apple to figure out.
I honestly believe they aren't. At least, not in the way you describe. Apple knows very well that the way people interact with tablets and with full-size PCs is completely different. They looked at just how badly Microsoft messed up when they did exactly what you describe: everyone hates Windows 8's "Modern UI" on PCs, yet loves it on tablets. And the reverse for Win8's "classic ui" (or whatever they call it) on tablets.
While Apple has been steadily bringing iOS features to OSX, they have been doing it in a way that still works for computers. In fact I think the
least "compatible", so to speak, way, has been launchpad which to be frank I think is completely useless. And I believe it hasn't gotten much attention since so they must agree on some level.
Also: you forget that iOS and Mac OS X actually do share a whole lot. Under the hood it's the same kernel running on both, and many of the libraries are either identical or very closely related. Where they differ is where they have to, to cater to the difference in devices and how they are used.