Being irreplaceable at work gives a false sense of security. Job insecurity creates defensive behavior. Working to be irreplaceable alienates you from others. You create more distance between work and result. People around you will try their best to ensure that not much of new work comes around you. When people relate you to a hoarder, chances of getting critical work becomes closer to zero. This stuns your opportunities for growth.
Let go being irreplaceable. Focus on being capable.