Even if WotC's servers go down, you could use the blockchain to prove ownership of a card for use in a live tournament. It's also possible for a third party to create their own server to implement the game, and then refer back to the blockchain for ownership.
As for non-official cards, I mean, yeah? That's the case with physical Magic cards too. Not even actual counterfeits have held the MTG secondary market back from astounding prices. Meanwhile non-NFT cards would be the equivalent of proxy cards - cards that aren't X, but both players agree to treat as X - which haven't been any more successful than counterfeits in holding Magic prices down.
I'm not claiming there would be greater faith in a blockchain than WotC, I'm just saying I see it as possible.