Someone else's bonzai tree
Especially relevant in big companies. When you develop an intricate, sought after skill set, but never rise to the level that was possible from your early potential, that may have been/still could be achievable elsewhere.
Zach has a PhD from an ivy league college, and has built enviable experience, but has never risen above director level in the blue chip company he works for. He's become little more than someone else's bonzai tree, and should probably leave, in order to become a red wood.