One of the great things about being at the same company for a more than a few years is having the experience of doing it all; if you're lucky. You were there for the good and the bad - big time client, re-organization, promotions, demotions, on so on. You have also been there long enough to know everyone's job and possibly have done some of each job because the moment called for it.
Knowing how to do everything and doing everything can give someone a feeling of importance. All the new employees at some point are pointed in your direction for direction. Officially and unofficially the manager has come to rely on you. You can give detailed instructions on how to do something and all the scenarios of why and what can happen if any piece of the action is missed. There is definitely clout given to someone who knows more than the average employee. So what's the down side? You knowing everything and taking over because you do.
Eventually you have to know that knowing everything isn't really helping anyone. It's good that you know, so if asked you can give the background and a few "what if" scenarios that actually happened; but if you're not asked you should let the new person find their own way. With new people comes a new perspective and sometimes a better way of doing things. It's good for the growth of the company as well as your growth, since you have been at the same company for a while and don't plan to leave. Eyes from the outside world can be helpful.
So know when it's time to pull back. Don't wait until someone calls you a "know it all" or eyes roll because you can't answer the question without a story from the past. Sit yourself down without any fanfare, but knowing if something is needed you are the one they will call upon.
NOTE: Best Interview I've read in a while