I understand for other tangible assets, a WILL can be written to be revealed when the time is right. But for Bitcoin, I don't know how this can be done as it may require revealing the secrete codes of the wallets or storage device. Is there any other way this can be done?
This is what makes Bitcoin different: the private key and passphrase. Of course, no one can open it if we don't reveal the keys before your death. It's not a good idea to transfer only Bitcoin money if you die because the majority of them, they will sell all Bitcoins located in the wallet. So, if you think you are old, I believe now is the greatest moment to teach them about Bitcoin. They may not believe you at first, but as time passes and you continue to teach them, they will develop an interest in Bitcoin. If you find someone who is suitable for Bitcoin wealth, you can transfer it to him before your death and advise him on what to do with it.
Yes, it seems to me that if only the passphrase or keys are given to the inheritors they will not get the bitcoins. There may even be people who don't know about Bitcoin before you die. He does not know how to protect those bitcoins or how to use them for his personal needs. Then that bitcoin might be worthless as well. Because he might make a mistake there for which he might lose that money. They must be well educated about Bitcoin before giving them a private key or passphrase.