Is this consistent with other companies? Did Ford's heyday end after Henry died? Standard Oil after Rockefeller? Hearst after WRH?
Arguably Microsoft has faltered after Gates; what about Hughes Aircraft; well ... right. Founders cashing out after the equity event seem to be a constant death-blow to acquired properties; Snapple, CompuServe, Bebo, Mirabilis, Xoom, AOL, Excite, MySpace, GeoCities. But then again there's Paypal, reddit, and IGN.
Nintendo had Gunpei Yokoi, Atari had Nolan Bushnell, MGM had Louis B Mayer; Sony had Ibuka and Morita. Even Disney started tumbling after Walt's death.
Virgin has Richard Branson, Subway has DeLuca, Aldi has Albrecht, Koch has the brothers, Oracle has Ellison, Google has Sergey and Brin, FB has Zuck, News Corp has Murdoch, even HN has pg, Ubuntu has Shuttleworth, Wikileaks has Assange, Linux has Linus, and Wikipedia has Jimbo. Perhaps there's something to this on a more general scale.
There's other stories though; 3M, Kraft, Merck, Kroger, General Mills, Walmart, McDonalds, Procter & Gamble, Safeway, and Samsung; all continued success long after the founders demise or exodus and without quasi-government granted funding or monopolies (e.g. telecom, auto, and energy).
Although perhaps one could argue that these companies have either lost their soul or assumed a malevolent one; none of them, save Samsung and perhaps McDonalds really produce, I'd say, goods that capture people's imagination.