Complete waste of time and resources not only will you not benefit anything from it you will probably destroy your computer in the process after running it for 10+ years trying to get something from it. The only ones which you will be able to crack is the ones that are commonly used which have passphrases like "password" and you will find that there will be a ton of competition to take coins out of it. Look at some brain wallets which have simple passwords and see how much is deposited and when and then see how long it takes for the coins to be sweeped out. Its a pointless exercise.