I think they will keep using OS X 10.* until they make iOS the standard across all devices (A changed iOS that is...not the current...yet). When they finally merge the two OSes it will simply be called iOS * or some fancy new trendy name.
Will their products have more and more software features in common? Yes. If they aid seamless interoperability without detracting from capability. It's the ecosystem, baby! However, the operating environments for mobile and desktop are sufficiently different, and the demands placed on the devices are such, that there's little reason to make a one-size-fits-all OS. They'd still have to optimize the "common" OS for each kind of device it runs on.
Neither iOS nor OS X are the key selling points for iPhones or iMacs. Consumers don't buy "a device that runs iOS," or "a computer that runs OS X," the way that other smart phone and computer users look for "Android" or "Windows." The Android and Windows brands promise a reasonably consistent experience,
regardless of who manufactures the device - the common ground upon which consumers can cross brand lines in search of a particular feature or price point. "At least I don't have to learn a different OS!" Since there are so many competing manufacturers, that's a very important distinction to make.
In the consumer mind, the over-arching brand name is Apple, not iOS, not OS X. "Apple" promises a quality of user experience that crosses all product lines. It's very similar to the identity of IBM
until the advent of the PC. Businesses committed to IBM. IBM computers, typewriters, software, services
it was a package. "Nobody ever got fired for buying Big Blue." IBM tried to kill Apple and the other personal computer makers by adopting open standards, and ended up losing the "IBM-compatible" business altogether. Apple has far surpassed IBM, using IBM's old, closed-systems playbook from day one.
Both OS X and iOS are based on Unix, so you could say they've been "merged" since the very beginning. Why not leave it at that?
