You assume too much. When OS X was introduced, we were told to expect it would be Apple's OS for the next 25 years. That's been only 12 years so far, so we're only halfway through the originally expected lifespan for OS X. Don't expect anything to change just because Apple hits OS X 10.9 and you think they then have to go to 11. There can easily be a 10.10, 10.11, and so on (look at any modern program's version numbers, they are not decimals).
Point being, don't hold your breath for the mythical OS 11. It will probably never exist, as they would likely choose a different naming convention by the time it will be released.
jW