Who are these "big personalities" who "decide" folk?

I can think of a few big names, but their influence doesn't go much beyond the events they promote themselves. Even the biggest folk impresarios control only a tiny fraction of the folk scene, although the events they put on may be among the bigger ones. If you don't like what they put on, there are plenty of alternatives. Of course, it can be a boost to a performer's career to get booked for a prominent festival or concert, but I very much doubt that anyone wields "You'll ever work in this town again" power.

If anyone has "decided" the direction of folk, it is the innovating performers who inspire others to follow them. Martin Carthy, Nic Jones and John Kirpatrick, to name but a few, have helped to develop playing styles and approaches to song accompaniment on their respective instruments that have been widely imitated. But that's a by-product of their own talent and professionalism rather than a deliberate attempt to decide or control folk music.