in the case of BA miles, at least if you have a household account it's not that difficult to just keep the miles of your family member. Just don't tell BA that they have sadly passed away.

I must say, I think the attitude of any airline who treats the miles as an asset that can be transferred if specifed in a will is a very positive thing... it would be nice to konw that if I die that at least my partner would be able to carry on our passion for travelling (MFU'd of course)