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.
No, it wouldn't be possible because the concept of IP and copyright exists regardless of blockchains.
Even if all the WotC IP ends up with the most determined patent troll ever, the game itself could still be implemented.
As for the card art... who knows what the legal situation would end up being. But even if it's totally legally off limits, the ability to load custom art would render it practically moot anyway.
Furthermore, if said community creates tournaments and online services that require official WotC NFT to participate, that will further cement their value.
That you can create your own NFTs and your own tournaments isn't going to be of much use when the rest of the players only plays with the official NFTs and won't accept yours.
And if others are allowed to create their own servers so that the game can live outside of WotC's auspices, that would also undermine the value of the NFT cards, since anyone who is able to create their own implementation of the game could just as easily start creating non-NFT versions of the cards that people can use.