No doubt the question why Matthew read Isaiah 7:14 in the way he did, is
interesting. However, in that case we are dealing with the "historyof
interpretation", some 700 years after Isaiah spoke these words, and
notwith philology. It seems more appropriate to follow Jonathan's
suggestion to read the text first in context (litteral and historical):
Isaiah promised a sign to Ahaz, so if Isaiah would have referred
exclusively to a messiah 700 years later, it was a pity for Ahaz but he
never would have seen thepromise being fulfilled. It stands to reason
that Isaiah's words were meantto have a message for the people of his
own time....