Your mistake is thinking that it matters where the data is stored. But storing your trading card image file in the Github Arctic Code Vault just makes it more permanent, it does not change the nature of your NFT at all; neither does storing it on the blockchain.

The thing is, that the NFT is just worthless data as long as you don't know what's behind it. And if the people that currently do no longer believe in NFTs the nodes will disappear and your data stored on the blockchain is gone as well. In the end the blockchain is just a decentralised transactional database, I really don't get the fuzz about it. With proof of stake it even looses a big chunk of that decentralisation.

Doesn't the nature of the NFT have anything to do on the content to be found under that link?

NFTs are promoted with those images, and if that's unimportant, then what's the difference between an NFT and some random coin transaction?

Of course, but that content can still exist if the link is broken.

In fact, the file is itself only one if many representation of thr artwork. Reencode the jpeg, you get a different file, but we still think it and the NFT are linked.