When do you hold a child's hand? When there is danger, such as a busy road. Thus hand holding becomes associated with a fear response.
?????
Ummm...I don't know about you, but generally hand-holding for me and my child or other loved ones is a sign of affection. Like, if I see see two people walking down the street holding hands I don't think, "They must be terrified!" This whole line of pop psychology argumentation is goofy.
I've run a D&D summer camp and teach two beginner campaigns each term. Also, I've been teaching for decades. You don't teach people by "throwing them in the deep end," unless you are trying to teach them to hate something. You teach people by figuring out where they are at and providing scaffolding and modelling to bring them along so that they acquire confidence and begin to take over. You guide them (holding hands is an excellent way to help guide people, BTW), and then let them go.
With regards to teaching 5e, in my experience it is generally easier than other editions of D&D just because there are fewer inconsistencies in the rules, and the rules, though still extremely complex, tend to be more intuitive. As for when they are ready to DM: as soon as they want. That goes for making house rules, too: as soon as they want. It's only a game, and all that matters is that everyone is having a good time.
We have no shortage of DMs. If anything, we have a shortage of players.