I agree with most of what you've said. However, existing collaborative custody solutions such as Unchained Capital have, what I would consider, two major drawbacks:

1. If the customer posses 2 of the 3 keys (as is the case with Unchained Capital) then this does not protect against theft.
2. If the customer does not possess enough of the keys to not have to depend on the "bank" to withdraw funds, then you have the very scary possibility of the "bank" losing the key they hold for you and now you've essentially lost all of your savings. Another possibility is that they hold the key they hold for you hostage for whatever reason (think canadian government and the bank accounts they froze during the trucker protest).

So that leads to my main question: What method can be implemented to provide someone with the benefits the fiat banks offer while still maintaining the benefits that Bitcoin offers?