Well there's the traditional way of having a will but it's understandable that a third-party is involved if you have that since you need a lawyer to legalize transferring everything you own to your heir. If you want it to be more private there's paperwallet, your btc will be safe if you can physically hide the paper well and only your heir will be able to get it. It's a smart and subtle way of hiding your possession well.
You don't actually need a lawyer for that because as far as I know, there's common law and banking law that governs these things. Under common law, you don't have to "report" any cash found at home.
Imagine that you live with your mother and she dies. You are the only child and your father is dead or your parents are divorced. Whatever movables were there at home are automatically yours. If she had cash in a safe and you know the combination, it's yours. Nobody is going to ask you how much money, jewelry, or art pieces she had as long as there are no other heirs that can lay claim to these things.
If she had a bank account however, the bank will not grant you access unless you give them a court order.
With bitcoin there's no third party locking you out. If your mother left you private keys, seeds, or wallet passwords, the money is yours.
I agree with you that for a case of only one heir, a lawyer is really not necessary but in the case of a person with many heirs, it becomes a little complicated. Those suggesting a will are viewing it from legal perspective which is the formal way of transfer of asset in a situation where there are multiple beneficiaries. There could also be some other traditional way of doing this transfer like customs specified in some part of Africa and Asia... in the advent of death of a father, there is already a motherlity of sharing the assets defined by their customs. However, Bitcoin differs from physical assets and has its uniqueness and sensitive that requires some form of technical know-how.
A lot of great suggestions have actually been made so far and I tend to agree with those that have clear technological feasibility such as multi-signature wallet where the heirs will all be required to sign before access.
Nevertheless, in a case where the multi-signature wallet suggestion is not feasible, I am considering arranging my portfolio in different wallets according to the heirs I want to inherit them. As time goes on and when I guage their level of mental maturity, I could even give them access but with specific instructions never to temper with them until I am no more. This is because I feel it is better the rightful heirs access the funds than some third parties pulling a fast one on me and my heirs. This is just a possibility and I might still adjust it.