It can not be applied to Hezekiah whose rule was confined to Judah, and which was neither progressive nor perpetual.

Hezekiah had already been born when this prophecy was given.

Isaiah 9-6 CANNOT be a reference to Hezekiah as NO son of Hezekiah did or can establish the throne of David forever. The kingly line through Hezekiah ended and will not be reinstated.

It is also important to notice that Hezekiah did not nor ever will reign over the kingdom of David, David reigned over the 12 tribes of Israel. Hezekiah only reigned over the kingdom of Judah, a divided kingdom. A kingdom that has not yet been reunited under one king.

Hezekiah DID NOT establish the kingdom of David forever.

Hezekiah’s kingdom not only came to an end, it certainly did not end peaceably.
His kingdom was
destroyed by the Babylonians

The kingdom of David is given to Jesus and it is JESUS who will establish the throne of David forever.