Strange no one in this thread has recognized that the "X" in OS X stands for Unix. Not "Ten." Ten was implied, but that's a minor issue (and understanding of that fact was a major insider's point of pride in Steve's new company)
Most Unix variants ever released have cleverly incorporated an "X" or more letters of the word "Unix" into their own name (Xenix, AIX, Linux, HP-UX, QNX, NeXT, the Posix specification, etc).
Steve used this "tradition" to name and differentiate his new OS from the (old) classic Mac operating system. The fact that the last version of the classic Mac OS, was version "nine," and that the "X" signifying Unix also meant "ten" was just a great play on words/numbers that delighted him no end.
The Unix heritage was (is) the major significance of the symbol. There NEVER was a plan to have an "OS 11, or XI" as long as the architecture was Unix-derived. "X" = Unix. Period. Very important marketing tool to him.
So the first version was OS X 10.0. This confused the uninformed who read it as, "Oh Es ten ten point oh," rather than its intended, "Mac Unix 10.0." The slight confusion was pleasing to the Apple insiders.
And version numbers (the part of the name after the "X") increment as needed: 10.1, 10.2 . . . 10.11 . . . 10.ad infinitum.
If Apple stops emphasizing the Unix roots of its OS, say by omitting an "X" from the name, this may indicate the development of a new or hybrid OS meant to transition to ARM systems for all products. Although iOS is, famously, OS X.