I have a hypothesis. I'm not sure exactly when I started developing it (or when it went from a sneaking, subliminal suspicion to an actual hypothesis), but over the years I've come to believe more and more strongly that it's true.
It seems to me that any time you have a good, solid, working organization, there are one or two core people who are holding the whole thing together. As long those one or two people are involved, the team will hold together even if other members come and go (though there's also likely to be less turnover as long as that key person/those key people are in place).
Now, so far that doesn't sound like much of a revelation. In fact, it sounds like the opening line from any number of Business Leadership books and articles.
The thing is, the key people for any given team -- and this can be a company, a department, a church... almost anything, really, as long as it has some organization and some goals. The thing is, the key people for any given team aren't necessarily the ones in charge. They can be, but they don't have to be. They can be the ones who make the consistent good suggestions for how something should work, or the ones who help other people explain what they're trying to do so everyone understands it. They can be the ones who have a solid vision for what they're trying to do and how best to get there (which is usually, but not always, the formal leadership). Sometimes they're just the person who follows up and keeps track and makes sure everything gets done.
This is how you can change out a CEO without much changing the way a business works, but lose the head designer for the same company and have the whole business ready to collapse within a year. This is how you can have someone apparently innocuous retire (a good, solid employee, but nothing special) and end up having to hire two or three or four people to take over their duties. And this is how having one person leave can make an entire team completely dissolve.
It's not leadership, exactly; or at least, it isn't always what people think of as leadership, or identify as leadership. But in a lot of ways, it's actually more important.