Why these platforms don't use hash of a file inside blockchain as bare minimum. Shouldn't be too expensive to store that data? Hash would represent the "original" no matter where is it hosted on the future.
The way I understand it (and I might be totally wrong here) is that ipfs links do contain the hash of the file stored, so by putting an ifps link as the token URI you are both providing the content hash as well as a way to actually locate the content.