Isaiah lived in a time of righteous kings, with the exception of Achaz. Why was he foretelling a destruction? It seems clear from the texts that the cause of the final destruction was Menashe, who is not even listed in the opening verse. We find a foretelling of destruction in chapter 6, which seems like an inaugurating prophecy and according to Midrash, this was way before Menashe, at the time of Uziyahu.
Add to this the fact that our sages describe Chizkiyahu as someone worthy of being Moshiach, how could it have been that bad then?