Then, my friend, we must certainly throw away the words of this great prophet. While Hezekiah was a great king, without Messiah, he is dead, lost in his grave, his bloodline destroyed, and lost to time. If this prophecy was finished in his time, then Isaiah was certainly wrong.
For these words most assuredly did not come to pass:
Isa 9:7 Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
(e-Sword:KJV)
What do we do now?
This my friend isnt a problem...it doesnt mean that Hezekiah was to be an eternal being sitting on the throne of David. It does mean that the throne of David is forever. And when Messiah comes, there will be everlasting peace etc.