Well, prion diseases are very rare in humans (or at least their transmission is rare), and there is no evolutive pressure to create a super-prion that can infect more people, like happens with other pathogens. Thus, the most likely situation is that it will continue to be very rare until we are able to change it into virtually non-existent by technology.

They are a really scary thing, but it's a meteor strike style of scary, nobody really expects it to happen before we can deal with it.