As a guy who gets paid only because of his computer knowledge, the following paragraph is a bit of a comfort to me:
One of the many consequences of the exponential power-versus-time curve in computing, and the corresponding pace of software development, is that 50% of what one knows becomes obsolete over every 18 months. Unix does not abolish this phenomenon, but does do a good job of containing it. There's a bedrock of unchanging basics — languages, system calls, and tool invocations — that one can actually keep using for years, even decades. Elsewhere it is impossible to predict what will be stable; even entire operating systems cycle out of use. Under Unix, there is a fairly sharp distinction between transient knowledge and lasting knowledge, and one can know ahead of time (with about 90% certainty) which category something is likely to fall in when one learns it. Thus the loyalty Unix commands.
It's from Eric Raymond's book, The Art of Unix Programming in his "The Durability of Unix" chapter.