Jesus challenges the false assumption that you can grow out of the servant role; his teachings show we are created and saved to serve. Jesus uses everyday examples of serving. Our service may vary and will be endless. It's not only acts of service, but the mindset and heart of the servant he is interested in. Jesus is speaking directly to his apostles (most commited disciples) in this parable to caution them on how they think of themselves - so this should apply to all his followers.