I doubt that'll be true.

The Wii, 3DS, and Wii U's shops still let you re-download games you previously purchased and also download updates for new physical games you get. I imagine it'll be a similar situation with the key cards, where if you get one after the eventual shutdown you'll still be able to download it.

Would I still prefer a card with the actual game on it? Of course. But I suppose this is still alright.