One would think that making spells and stuff use attacks vs Fort/Ref/Will defense is equivalent to having Fort/Ref/Will saves vs save DC, but it isn't really. That's because they're affected by different things. In 5e at least, it's pretty easy to increase your attacks: Bless, Bardic Inspiration, assorted other abilities. But it's a lot harder to give your opponents a penalty to their saves.I don't know about bringing back attacking saving throws, but I would definitely favor bringing back Fort/Ref/Will for a variety of balance reasons.