I predict this change will be detrimental to elemental decks, especially Shaman with Servant of Kalimos.
Giants are strong in very specific decks with very specific synergies. Substantial discounts are necessary to make them worth it (or even play at all in arena).
Adding giants into the elemental pool will dilute the overall card quality for discovering elementals, since the giants will be unplayable in a deck not designed to capitalize on their discounts.
you could play an elemental deck and just not include giants if they dont fit your theme.
I bolded the important part since you missed it.
You can't avoid giants in the Servant of Kalimos discover pool. Molten Giant (in wild) and Mountain Giant don't work unless your decks are designed to make use of them, making them bad discovers. That being said, this is a fringe downside, and this change is for the most part a positive thing.