The concluding words of this parable, essentially repeating the conclusion of the previous section (19:30), tie this parable in with what has come before. Both the rich young man and Peter make the mistake of thinking that God deals with us according to merit, but the point of this parable is to show that God rather deals with us according to grace. His way of working is eccentric, turning upside down our normal expectations of how the world works.
I think we might well suppose that the workers hired at the eleventh hour, who have been “standing … all day long doing nothing”, had most likely not been hired because no one would want to hire them. They are the least deserving, not only because they only worked for one hour, but also because they were probably the least productive workers. By choosing to pay these men first, and paying them far more than they might have expected (but what they needed – for a denarius , as well as being a normal day’s wage, was what a man needed to provide for his family), the landowner would have raised the expectations for those who had worked throughout the day and so could deliberately challenge their (and our) thinking.
We readily sympathize with the workers’ complaint, because we normally expect remuneration to be proportionate and fairness to be tied in some way to merit. That’s the way the world works, but that’s not how the kingdom of heaven works. And when we bring those kinds of expectations into our thinking about how God treats us we easily end up grumbling at our lot (v.11) and envious of others (v.15). Such grumbling and envy betray an underlying assumption that God owes us and is not being fair. But God is not unfair (v.13), since no one gets less than they deserve, rather God chooses to be generous to some – to give them much more than they deserve (and exactly what they need). Such grace is an aspect of God’s sovereignty (v.15).
Though in reading the parable we are all inclined to identify with the workers hired first, we ought rather to identify with the 11th hour workers, knowing ourselves to be undeserving, and grateful for God’s unexpected grace.