One problem with sticking around is that the number of things you had a hand in, and will therefore be consulted about, grows unbounded. Being peripherally involved in a wide range of initiatives can give you a kind of global view and influence that dovetails nicely with certain visions of a "senior" or "staff" role. But it does come at the expense of being able to go deep on something new.

I've been with my employer for much longer than average. They're taking care of me well financially. Even the variety of projects and domains is pretty good. If I leave, it will be to declare bankruptcy on being an advisor/carrier of institutional memory.