MAD was always criticized, but that criticism becomes more and more valid each year. There's too many options and opportunities on the field. A Second Strike is not guaranteed in the modern world. There are countless examples where soldiers or others in the chain of command will not obey a "destroy the world" order.
I'm not saying any country should take the gamble, but there are enough ways to put your thumb on the scales that a nuclear solution against a nuclear power could become feasible (if genuinely terrifying) in many hypotheticals.