No. It is just that we do not have a cure or vaccine because it does not appear that often. And no, prions will not eradicate entire species because they not living under any definition. Therefore there is no direct evolution, if you are resistant against an outbreak, your are resistant against it. The prion cannot develop to overcome this resistance. Essentially, a prion disease is very like the computer virusses we have today: They do not change their source code, so fixing the issue in your code will disable them.