Every really large and powerfull organization eventualy stops excelling.

I think it has to do with management by metrics (in most of the older cases, the terminology is different, but the concept is the same) rather than by people focusing on the real thing.

In a new organization, people know what it is about, and measure performance against what it really is about. But eventualy, the organization is too big (well, it's how the people at StoneDampHole explained it to me back when I was a productive member of society) to focus directly on the real goal, the big goal, and it becomes neccessary to measure performance against weird little proxy goals (metrics). And in many cases, the organization becomes real good at making the metrics come out nice. For example, StoneDampHole decided to use RONA (return on net assets) as the key metric. I'm sure that, having dumped most of their assets (including me), SDH has a lovely RONA. But who the hell cares?
Another example: the Catholic Church decided that membership numbers, particularly numbers of nations, was the way to measure excellence. And thus we have, well, European history from about 300 to 1776. A crusade is great for numbers, but not at all what that transient preacher with the authority problem out in the boonies of the Roman Empire had in mind.

A really good clue, I think, is when people in an organization have to figure out how to measure performance. If it isn't obvious, it's time to fold up the tent.