I'm not surprised at all. But, a man can dream. And it is perfectly possible for a card that is relevant to Legacy to be printed that just isn't good in Standard. Past In Flames, Young Pyromancer, and Omniscience both come to mind as cards that meet these criteria in the not to distant past. Miracle as a mechanic is another that was certainly viable in Standard but not over the top broken that basically reshaped Legacy by making what is plain and simply the best control deck and when competently piloted perhaps just the best deck in Legacy.
As for Nahiri, she isn't objectively bad, but in Legacy Miracles she would be fighting for the same slot as Jace The Mind Sculptor so obviously she loses...