Prion caused disease is indeed terrifying on individual level but, from its impact over the species' survival, it isn't that menacing. The decay takes decades, which is enough for breeding the next generation. Human life expectancy may get shorter, which sucks, of course, but humans lived shorter lives for most of their history. So no, it's not that bad. Even assuming that the entire human population will get infected from young age with no way to avoid it, it won't be that bad. It would come with a bit of adjustment, like having to raise kids sooner because the luxury to postpone (for life enjoyment or other pursuits) won't be there, but things will go on.
