It’s ability to be stolen really comes down to was the password to it ever written down or did they not use a password.

If they didn’t write it down, you could conceivably keep it safe from theft as long as the people that know the password don’t give it up if tortured.

Also breaking it up into multiple wallets with different passwords that not everyone knows would limit a complete theft.

Edit: in the scenario where it can’t be stolen, killing everyone that knows the password would still leave the country with nothing though.