The allegory returns to the original tree. It has produced much fruit, indicative that it is thriving. However, when the Lord of the vineyard tastes the fruit, it is not good. It has the appearance of prosperity, but is still useless for its intended purpose.
The real-world sense of the allegory now moves to a time when Christianity has been grated into the covenant root of Israel. Christianity, in particular, has prospered, and the Jews have also become more populous. In numbers, the people who have inherited the covenants are large. For the Lord, however, it is not the numbers, but rather, the transformation of souls that matters. Individuals are the fruit of the tree, and the ultimate goal is to transform the fruit into something more pure and more important. For the olive tree, it was the production of the oil. For God it is the refinement of the soul.
The long time that has passed has led to a false impression that the tree of Israel is doing well. It is not. It is not producing the quality transformation that is desired. Thus again, something needs to be done. Even with so many failures, the Lord of the vineyard continues to attempt to save the tree so that he might have the desired fruit.