Big fish are almost always eaten by small fish. On the other hand, almost all small fish fail. It's the rare exception among small fish that bests a big fish. Stated correctly it sounds unremarkable because it is unremarkable. So much for "Leslie's Law".
His DEC/IBM comparison shows that he somehow stopped reading the tech press around 1990. Intel's decomposed platform ate them all and opened the door to Linux and open source. Intel still dominates servers, but on the client side has been bested by the system on a chip cpu designs licensed by the mobile vendors. The two big client OSs are of course rooted in open source (iOS is from OSX is from NextStep is from Mach/BSD, and of course Android is Linux).
EDIT: Lawtonfogle is absolutely right, so I removed the word "completely" and left "unremarkable". Good point. Also I think most small fish die on their own. Usually because they build the wrong product.