The mainstream public will never self-custody not because writing 12 words on a piece of paper is too difficult but because physical defense against $5 wrench attacks, mental defense against scams, and cyber defense against hacks are too difficult. (Yes, you don't have to defend cold storage against hacks in theory).

Most importantly, mainstream public treats banks and governments as their friends, although imperfect, and doesn't understand why people have to go through all the trouble and responsibility of being their own bank when banks already exist.