In order to do that you'd have to re-hash each of the data-dumped passwords with a new salt, each time you wanted to do a comparison. But a good password-hash a

In order to do that you'd have to re-hash each of the data-dumped passwords with a new salt, each time you wanted to do a comparison. But a good password-hash algorithm is specifically designed to be slow, so re-hashing the whole database will take quite a lot of computational power, likely enough to make that strategy infeasible.