Encrypted of course. It can never get lost, stolen or destroyed.
If you want to store bitcoin for someone 6 generations down.
How else would you store bitcoin for 10k years?
Companies, clouds, hardrives will disintegrate but the block chain will always be there.
Edit: I meant storing your private keys on the blockchain
A private key is essentially a very secure "password" that allows you to access your bitcoins. Anyone with access to this private key has access to the bitcoins, and anyone without access to the private key does not have access to the bitcoins.
Your plan is to take this very secure "password", and encrypt it with a weaker password. Then store the encrypted private key in the blockchain. In order to access this private key (and therefore access the bitcoins), you would need access to the password that was used to encrypt it. How would you keep this new password safe? How would you get this new password securely to the person 6 generations down? How would you keep this new password secure, and get it to the right person 10k years from now? Couldn't this new password get lost, stolen, or destroyed? You couldn't count on any companies, coulds, or harddrives to keep track of this encryption password for you since (as you state) they could disintegrate.
Perhaps you could encrypt the password that you used to encrypt the private key, then you could store the encrypted password in the blockchain! Oh, wait. Now you need some way to safely store the password that you'll use to decrypt the password that you'll use to decrypt the private keys.
Perhaps you could encrypt this new password and then store it in the blockchain! Wait. No. Now you need some way to safely store the password that you'll use to decrypt the password, that you'll use to decrypt the password, that you'll use to decrypt the private key.
I think I'm starting to see a problematic pattern here. Do you?