Finishing the Task will not bring Jesus back - in the sense that it will force him to return (or, inversely, not finishing the task will not keep Jesus away).
I recognize that some interpret Matthew 24 very differently - as a prophecy more to do with the fall of Jerusalem in AD 70. But while I want to avoid the "tying-together" issues, I also want to remind us that Jesus himself tied together the two ideas - "the Gospel will be preached in all the world" and "then the end shall come."
As far as I can see, the end has not yet come, and the Gospel has not yet been preached in all the world.
I rest assured of Jesus' words - which are as much promise as prophecy. These two things will come to pass: the Gospel will be preached, and the end will come. But they may not happen soon, in our lifetimes, or on our timelines.
We are on God's timeline. He is patient with us, not wanting any to perish. We must be patient, too.