Going mainstream
The success of the local PP has made the established pollies realise that they can't fob people off with a "trust us, we know what we're doing" and they're picking up on the issues the PP have had to themselves. The partial release of the ACTA text is a good example of something has happened internationally. The result is that many people don't really see the point of the PP any more.
I guess that is the primary rôle of a single issue party, but I hope they don't die out completely as there's still a lot that needs doing.