All bitcoins that are not moved for a certain period of time (20, 50, or 100 years) should be presumed lost and then recycled through the usual mining process. I'm not sure if that can be achieved through a soft fork.
Math/cryptography tells that they belong to somebody. Even after 100 years. And that means
theft.
You know that one can inherit a private key, a hardware wallet or a physical collectible with private key on it, right? You want to make them worthless in 100 years. Unless the term property will become more.. lax.. in 100 years, I don't see this happening.
And, more or less as philipma1957 said, I'll leave this for my
grandchildren think it over.
Which is my main point it is going to be down the road a bit before that fight happens.
In fact if it does happen it would mean 2 very good things.
Both the world and BTC survived long enough to have that fight.
To all that disagree look into sunken pirate ships and the treasure involved when found some very interesting laws are involved as to who gets to keep what is found.