I completely agree, thank you for sharing your experience.

It's a challenge for some experienced developers to share their knowledge. They take the weight of the whole project on this shoulders which mean that other developers don't get to learn from hard challenges.

I have complete sympathy for it, I've done the same. It can feel like you don't have the time to train other people so that they can free up your time.

Like many long-term multipliers, it costs short term time, but hopefully pays for itself in the long term.