What Bitcoin and other cryptocoins need, especially for things like inheritance planning, is an auto-transfer feature to transfer all your bitcoins in different accounts/addresses to a single address your next-of-kin has access to, when the other addresses are nolonger active for specific period of time..
So, your next-of-kin (could be your wife, mother, father etc) is intially given a copy of the keys (seedphrase) of the address that will recieve the transfered coins when you are nolonger alive/accessible. The next of kins hides the keys, and you could also have a copy just incase the next-of-kin loses hers or just in case the coins are transferred while you are still alive(maybe the accounts were inactive due to ill health, imprisonment, forgotten, etc).
But since this feature is probably not available for now, bitcoiners and others could simply write down all their keys or seedphrase with bitcoins, on papers or metallic sheets, with clear instructions on how to use them to access the bitcoins... Then lock them up in metallic safeboxes and hide them somewhere safe for their next-of-kins to access when they are nolonger alive/accessible... you could have dublicates in multiple boxes hidden in different places. The next-of-kins should be aware where they are hidden. And then give the physical keys of the boxes to the right churches, institutions, etc, for safe keeping, on the condition that they get released to your next-of-kins who present your authentic death certificates or things like that.