Train someone so well they could take your spot. That’s not a threat, that’s a promotion waiting.

Keeping someone slightly underdeveloped to protect your position is a strategy that only works if you never want to move. Being replaceable is the actual requirement for promotion, because nobody can move you up out of a role that collapses without you. Beyond the practical side, a person you trained well will speak about you for the next decade in rooms you are not in. That is worth more than being needed.

Teach somebody the part of your job you have been quietly keeping to yourself. Let them do it badly once.