Fiat could survive another 10 or 15 years, while Bitcoin price could swing around wildly. If they have enough Bitcoin to make them wealthier than they were before (arguably anything over 1%) then maybe that's enough risk for them.

And another question is how old is this person? If someone is like 75 then they should only have a small portion in Bitcoin anyway.

Going all in on Bitcoin is for young people who aren't rich already. (In my humble opinion.)