There are many ways Blizzard can make content relevant outside of gear. Karazhan could come equipped with a mount, toys, pets, transmog, or even a new illusion skin. They could add a pet battle somewhere in the dungeon with unique rewards. All of these things would keep people coming back to the instance well beyond its initial lifespan. Not everything needs to have gear attached to it, although that seems to be the trend with most new content these days.