The idea of working for naught would be awful! Even as we follow God's leading, sometimes things appear to be this way - no reward, which leads to discouragement. What about Jesus? He lived only to die among unbelievers, to pray among sleeping disciples, to heal among death threats. He had no reward - not immediately.
Indeed, this is He of whom Isaiah speaks in this passage. He came because He loves us - and still comes to the unbelievers. Failure is not going to happen, for the work is the LORD's. He called Isaiah before birth, and Jesus from the beginning. Something will happen, for God is guiding and working through us! He knows the end.
Soli Deo gloria.