That's a valid concern, I think. But so far it seems more likely that the exchanges will belly up, through mismanagement, exit scams and the like. I am a bit concerned about the mining aspect on the majors that use that method. I'm not certain it is sustainable, but one thing I've noticed on the blockchain is someone, somewhere seems to come up with solutions to problems before they get too far out of hand. It reminds me of the movie Avatar, a sort of global hive mind at work. I find my faith seems to be resting in that community hive mind, even more than in the tokens and coins that fuel them.