I somewhat agree, and I think it's largely already happening. For the most part I think it's unlikely anybody who's not somewhat tech savvy would ever move their coins off of a service/exchange like CoinBase, and such an exchange already provides some of those guarantees and could easily provide more. All the benefits and drawbacks of bitcoin pretty much go away if you never actually use any "physical" bitcoins and just go through an exchange. That said I have my doubts about it ever actually getting all that big for everyday use, everybody is always going to need dollars so I don't think there's all that much of a reason for big banks and payment systems to "switch".