I think that elliptic curve cryptography was invented in the 1980s (Wikipedia), so 75 years ago is well before that. I imagine the rate of change has picked up a bit, so it seems highly likely that the cryptography we use in bitcoin (or perhaps any current cryptography) will be much less secure in 75 years than it is now.
So if you bury it like pirate treasure, the problem is that the blockchain is never buried. And as vulnerabilities are exposed (or computing capabilities increase) it's likely that people will be treasure hunting long before you have a chance to unearth your coffee can.
I suppose the "at least" is a minimum.
I think that elliptic curve cryptography was invented in the 1980s (Wikipedia), so 75 years ago is well before that. I imagine the rate of change has picked up a bit, so it seems highly likely that the cryptography we use in bitcoin (or perhaps any current cryptography) will be much less secure in 75 years than it is now.
So if you bury it like pirate treasure, the problem is that the blockchain is never buried. And as vulnerabilities are exposed (or computing capabilities increase) it's likely that people will be treasure hunting long before you have a chance to unearth your coffee can.