I would like to draw a distinction between the goal and the reward of a thing. The goal is the purpose, the it serves. The reward is the "why," the reason we do it. They are usually looked at as the same thing, but they can be distinguished, at least, and separated.
First, what is the goal of missions? To change lives and cultures so that they image God's love. To bring lost people to Christ. Good things. We go to preach the gospel which justifies, sanctifies, and ultimately glorifies. No surprises here.
Second, what is the reward of missions? That is, why do it? If we do it for the sake of changing people's lives, bringing them to Christ, then what happens when it looks like no one is listening? You may never see the fruit of your labor. You may plant, but never know it. If God were to call you to serve, and told you that absolutely nothing would change on this world, how would you respond? Thus, the reward of missions must be separate from the simple aim of it. The reward of missions is the kingdom of God, that is, God's glory. Whatever we do, we do for the glory of God, that is the reward. If we ask merely to see the fruit of our labors, as great a reward as that may seem, it pales in significance compared to knowing God, my rock and my redeemer.
We might not reach the goal ourselves, for we are not in control of that. We may only step out in faith, knowing that our reward is in heaven. Thus we are invincible to the arrows of failure, for we are in Christ, and so, for us to fail with that finality would be for Christ to fail, yet he overcame all things, even death, that we might live in his resurrection. Thus, as Christ loses no one, so, as we are in Christ, we do not fail to save any. This is how we have a reward which is kept for us in heaven, whatever becomes of the goal: Christ is victorious over sin and death, thus there is nothing more for us to conquer.
Go and make disciples, and Christ is with us always.