Yes, that’s true, however you cannot do that. The contract for that data has it’s name in it. That means you can only store the exact bit of data you paid for. As name == content hash then there is no way to game the system.
For mutable data we still need to work out details, but again I am keen those are all 32 byte fields and you pay per field. That keeps cost even and also allows the contract to state which field you are paying for. So again no gaming there either.