Whatever, but if I understand you correctly, that still doesn't the situation for the great grandson right? He finds the chest with 100 million USD worth of gold after 110 years and it is not his anymore? Whether someone + their successors arbitrarily decide to keep it for more than 109 years or whether is lost doesn't matter? I mean what if I just don't want to sell but keep it as part of my diversified portfolio? What happens then after 109 years?
It is not that I don't get the problem you are describing, but it is a very delicate topic as it goes straight to the core fundamentals of Bitcoin which as of now essentially are the value proposition.
A wallet can "verify" its activity by a single transaction, of whatever entity. If the wallet doesn't make transactions after a given amount of time (I've proposed 131, as the full mining cycle is) the coins are recycled into the network.
There is no threat to holding coins, nor to keep diversified portfolios, or threats to legitimate inheritance.