Scott Forstall recently testified in court (Friday) that OS X was designed to last 20 years.
As it's only 11 years old now, and I don't see any OS that threatens its position of overall superiority, I'd say the odds are good that we'll see 10.10 and possibly beyond.
You can bet, however, that somewhere -- in a dark, never-discussed portion of the Apple Campus -- a small team is busy throwing out EVERYTHING we know about OS X and iOS and starting over ...
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Once your machine has gotten to the age that it only meets the bare minimum specs on the latest version of OS X, you probably have a choice: upgrade some things (RAM, HD, software) and keep up one more time, or (particularly if everything is working well) stick with what you have.
You can continue to upgrade the non-OS software for as long as you eligible, and Apple issues security updates for some (just the last couple of) older system versions.