Oh boy, here we go with another person cherrypicking that one verse and using it as "THE STANDARD". To me that just shows ignorance and no real understanding of judging prophets and prophecy.
Isaiah was one of the greatest prophets in the old Testament, the prophet that gave the most prophecies about the coming Messiah, the prophet that the Messiah himself quoted.
Isaiah prophesied to King Hezekiah "Thus says the Lord: Set your house in order, for you shall die, you shall not recover.". No conditionality to that word. Hezekiah DID NOT DIE, the prophecy DID NOT come to pass. Should Isaiah be stoned to death? Was he a false prohet?
I call this the "Cherry Picker's Isaiah problem".
Really? Is he still around?
Indeed, you refer to a conditional prophecy when it comes to Hezekiah; and so many prophecies are and were conditional. (Although interestingly enough, Hezekiah did die after the additional fifteen years . . .)