I think a hardware wallet with a secret passphrase is the best self custody solution for most. Jade is excellent for this. They need their device and a pin. Same set up asost bank accounts... You have your debit card and your pin. 

And it's not about managing a single key, it's about managing multiple keys. First off, the concept scares most away and I think the complexity increases the chance of failure.

For some, custodial solutions will be the best whether the Bitcoin community wants to accept that or not. People want simple.