I've always thought that silence--meaning here the absolute absence of the solo instrument or all instruments in an emsemble playing for some duration--is just another musical element. For example, the Toccata from the Toccata and Fugue in D minor for organ usually attributed to J.S. Bach (the prototypical piece of scary organ music) is full of silences. Given that, my take is that Cage was doing something similar to what the color-field painter Ad Reinhardt was doing: making a work from a single element. Now Reinhardt's paintings weren't absolutely of a single color: there are slight variations in the overall field of reds, blues, or blacks he used that makes the paintings seem to shimmer or breathe (and even makes people who sneer "My kid could do that"—when, of course, their kids have nothing approaching Reinhardt's command of color—stare deeply into those paintings.) And so Cage's comment about there being no such thing as silence comes into its own.
smg