If you have a cold wallet, all you have is a key. You can't ask a node "how much bsv do I have?" It has absolutely no idea, nor has it ever. 

Currently block explorers build an index of which address owns each output. That requires, at bare minimum, 56 bytes of storage for every output which has ever existed on the chain. You could trim that to only UTXOs, which could still be trillions or more. Assuming someone does keep those an make them available, you still have absolutely no way to verify that they really gave you all of your UTXOs. You are at the mercy of someone else to tell you what you own and to make those outputs available for you to download so that you can spend them.