Percentage of people holding it in 15 years, or from the people that hodl today?
If it's the first, I'd say less than 1%...
How many people run their own servers?
And with tx fees at 50$ (or more, in 15 years), I don't see much self custody.
Also, if one has a utxo of such value in their cold storage, maybe think about consolidating before it's too late, otherwise it will be useless.
Imagine the "useless utxos", those of less than the current fees. Has anyone counted them?