You think HPMoR won't be? It has the kind of intensely devoted (not to say cult) fanbase that preserves things and actively recruits new fans. If your view is that Harry Potter itself won't be read 500 years from now, even if that were to happen it's reasonably common for parodies to outlast the original. If I had to bet on a piece of literature from the last 10 years (say) that would last 500 years HPMoR looks like one of the better options.