You can only know for sure that someone is in possession of a private key when coins are spent, but large amounts of unspent coins could create uncertainty to trust the system because they have influence on the trade markets when they are spent.
So I was wondering, when do people agree to be 'sure' that coins are lost forever and not stored in a savings account?
My first guess would be the average life expectancy, roughly 77 years.
Average is 77 years for a life but we are only adults or in control of ourselves for just over half that period. If bitcoin hasnt moved out of address for 40 - 50 years its not looking good that anyone can move it. Obviously there will be exceptions.