The judgment would come, without a doubt. Isaiah emphasized that these were not his own words. The meaning is not that the people would atone for their sins through their death. No one can atone for their sins by their own death! The statement means that there would be no pardon for this sin. Turning your back on your Saviour leads to doom.
14 The LORD of hosts has revealed himself in my ears: “Surely this iniquity will not be atoned for you until you die,” says the Lord GOD of hosts.