Well, nice question. I personally think that their bitcoins will be put in vain. If no one has access to it except you, then no one can after you die since the system does not know if you are dead or not. The best way I can think to prevent is to have a trusted friend who also uses bitcoin, or any of your family members. Let them know about bitcoin and let them earn in their own way so that they do not have to open yours then if you died,all of your wealth will be transferred to your family.