‘So the last will be first, and the first last.'
And thus the story tells us that because of God's goodness and graciousness, and because our spirits can so easily become jealous and hardened, it is often the last who become first, while the first become last. This is a warning, not a threat. The sad thing in the parable is that it was the men who had worked hardest who came out worst, not because they were not fairly paid, but because they were ungracious and mean-spirited and finished up dissatisfied.
It is interesting how often commentators at this point cite stories where a man who only worked a short time did as much in that short time as those who had worked all day. It emphasises our sense of fair play. But that is almost to cancel out the point of the story. For the point of the story is not that we get what is due because of what we have accomplished, but that if we have done our best God is so gracious that we all get far more than we deserve, regardless of how much we have done. The point is that God is generous beyond deserving to those who seek to serve Him and that we should not be looking at what others get, but wondering at His graciousness in giving us so much when we are the least deserving.
For the real emphasis of the story is not the workforce, nor on what they received, but is on how we should conceive the goodness and graciousness of God, and on the fact that we will all come out of His vineyard with far more than we deserve, because of how good and generous He is. It is that our rating does not depend on what we deserve, but on His goodness alone. Once again they learn that the new world is upon them, a world unlike any known before, a world where the only criterion is the good, and where men receive far more than they deserve. (In fact, of course, God had always been like this, but now it is revealed as the very basis of the new age).
Thus the idea that ‘the last will be first, and the first last' warns against presumption when we are dealing with Someone Who is the very opposite of all our reasoning, because He does not think in terms of what we deserve, but in terms of love. Thus none can set himself up above any other, and the Apostles least of all. If this was not intended to prevent the Apostles getting the wrong idea about their ‘thrones' we do not know what else would have been. And shortly we shall learn how necessary it was (Matthew 20:20-28).