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.