There are two problems that contribute to mission drift. Well, there are more than just two, but there are two that represent, as Newton might put it, were he here, opposite and equal errors.
One is the error of turning over the reins before the vision is deep in the institutional bones, and to turn it over to those who have been around long enough to parrot the vision (which takes about three weeks to learn), but whose actual function is parasitic. The energy and strength of the organization comes from the founders of it, and one of the techniques used by leeches is to live off that energy and strength — the kind of thing they cannot generate themselves.
But the other error is that of being so afraid that the next generation of leadership will “not have a clue” about the vision that the leaders cling to that vision with a death grip, and take it to the grave with them, the vision having deceased about twenty years before they do. The vision ceases to be whatever it was, and becomes the perpetuation of them being the custodians of it, deceased or not.
The alternative is to recruit and train young blood over the course of some years, and make sure they are being recruited for positions of real responsibility and authority.