Isaiah 7;14 isn't discredited.. it just isn't talking about a virgin, and isn't talking about Jesus. If you read it in context, you will see that the young woman (almah) is the prophetess,and Isaiah's own wife. The son is Isaiah's son, and the sign is that before Isaiah's son can be old enough to understand good and evil , the king of assyeria will be defeated
That is an immediate sign to King ahaz, not something 600 years in the future.