Well - its something to solve ;-)
You could use a smart watch (if it looses your heart beat it contacts a person and that person confirms your death to the watch, that then triggers the smart contract)...
Or our governments could issue a key for each citizen and some kind of multi layer witness-confirmation scheme could be implemented that at the end triggers the smart contract.
I mean, its what is being done now, right? So who ever issues the death certificate today could also have a key to contact "last will smart contracts" and inform them about the death...
Just depends on how "Libertarian" and "Tech-loving" you are - but it could be solved ... using a mix of today's solutions and a chain ;-)