if it hasn't already been used to steal ALL block chain material
No need to steal the block chain material.
Just become a node and you have access to all of it to download
Then you can have your magical quantum computer crack all the passwords and encryption and transfer all the amounts to addresses that only you control.
Just one more thing.
Now you just have to convince ten thousand+ other nodes that still have the original blockchain to all switch to your new blockchain with all your changes.
No need to steal the block chain material.
Just become a node and you have access to all of it to download
Then you can have your magical quantum computer crack all the passwords and encryption and transfer all the amounts to addresses that only you control.
Just one more thing.
Now you just have to convince ten thousand+ other nodes that still have the original blockchain to all switch to your new blockchain with all your changes.