OS X will be around until there's a need to replace it. I don't see anything on the horizon just yet that will demand that change.
OS 9 got retired because it was ancient in terms of architecture - not multi user, which made security it for internet use pretty hard. No memory protection, co-operative multi-tasking, etc.
All valid trade-offs back in 1984 when memory was limited and CPU power was minimal, but a huge liability when trying to secure a machine in a hostile networked environment.
OS X was massively ahead of its time when released, and is still at the forefront today. It has proven portable to multiple CPU types, is endian independent, supports fat binaries, etc.
Unless there is some revolutionary new UI paradigm invented, I do not expect OS X to be retired.