10.9 = Last needed cat name.
11 = New naming scheme that doesn't involve big cats.
The final release of 10.4 was 10.4.11.
But you are absolutely correct, there is no reason to assume it won't happen.
Interestingly, the Engadget article mentions more than once they thought 10.7 would be the last 10 before the jump to 11.
"Right now, at least, the company's not ready to close the book on chapter X"
and
"This may not be the jump to OS 11 we were expecting after Lion"
10.9 = Last needed cat name.
11 = New naming scheme that doesn't involve big cats.