Interestingly enough this is a very common misconception. The fact is that NFTs are nothing more than just another useless token that in fact have nothing to do with "digital art, etc.". They are just selling these tokens with a promise that there is a link between them and whatever digital stuff you buy from them, usually it is a hash to create that link.

Now with that clarification you an clearly see that if that centralized server that stores the digital goods goes offline that NTF is not going to do anything for the owners since they no longer have their "goods"!
Another issues is that there is nothing stopping others who don't have the token to get the same goods! For example someone buys a so called "digital art" for a lot of money through an NFT and receives the "art" but also anyone else who has not spent a dime would also get the same "digital art"!