Anything you keep in a drawer for emergencies will be used in an emergency, and that is what drawers are for.
Had the scrolls been sold, the old life would have converted into capital and stayed alive as an option in the mind of the seller. Burning them where everyone could watch shuts the way back with other people's memories as well as one's own, so the new identity holds on the night the old skill would have been useful.
Saying the sum out loud fixes in the body exactly what was surrendered, so nobody in that crowd could later tell themselves the change had been cheap. Burning costs a person the comfort of knowing the old trade is still there, and that comfort is the engine that keeps old lives running.