Comment by jebarker
My pet theory is that this is the natural lifecycle of almost all companies and the reason for that is that they underappreciate the luck involved in their first success. There are a few exceptions in the form of zombies (typically relying on a monopoly or legislative help), but there are very few repeatedly innovative companies.
There was another thread (I think on HN today) about investment strategies, and the ones that earned the most over the longest term were basically broad index funds rather than picking winners. I'd wager your point has a lot to do with why this investment strategy is best.