Please allow me to interject here. I will be upfront that I am a windows user and I have a bias. That being said, a long time ago prior to either system was this thing called UNIX; the first true windows type system. There was an open source version of this back then called Linux which is still around today. There was also competing system to UNIX which was called BSD which had an open source version as well called FreeBSD. What's important to note here is that yes, Microsoft ripped off Linux, but Apple ripped off FreeBSD too. Both companies were culprits in that regard.
Since then, Linux continues to be a robust system that offers a array of capabilities and expandability and is in continued development. FreeBSD however does not and fact is, BSD fell out of favor when the last major update occurred in the late 90's. Since then, BSD has all but faded away as UNIX/SOLARIS/WINDOWS system are far more capable and suited for running large cooperate networks and high level computing while both BSD and FreeBSD lacked needed expandability due to limited code. UNIX and Linux both run 32 bit, 64 bit and 128 bit level computing while BSD and FeeBSD only has limited 64 bit computing abilities and no support for 128 bit. You may ask why this is necessary and the answer is higher level computing means more capacity, faster processing and strong data encryption.
OS10 which was release shortly after the latest release of FreeBSD in 1999. Should say a lot that is 17 years, mac has not seen a major version change, plus changing the number 10 to an X was nothing more than a marketing scheme to keep what users it had engaged.
Speaking on my background as a computer science major with programming background; software versions are broke down like such 10.1.1. 10 being the major revision/base system. The first 1 being the minor system. The 2nd 1 being minor subset system. In software terms, whether your still using 10.0 or 10.9, your still on the same major system version with minor changes. Additionally because BSD has been obsolete for nearly 2 decades now, I foresee the only way for OS11 or any other marketing scheme that apple using to lure you in to purchasing, will undoubtedly have to replace its beloved Joshua code altogether. If they release this so called OS X 11, please keep this thought in the back of your mind when dropping the 2 grand on that new machine, "Your buying the same OS you bought in 1999 with a new wallpaper, new color scheme and new cat name". The heart of that new cat, unfortunately is the same at the one on 10.0. In essence, 2k for a new/old system.
iOS however, for those of you who think it is a completely different system should guess again. iOS is a port of OSX. A port being minimized version that's optimized for portable devices including limited processing power and expandability. Soon, even it will reach the same end of life as it is based on a system that is 2 decades old.