As the cross-grafted branches grow, the branches that that are not good will be cleared away. This is the winnowing of the wicked in the last days. Verses 68 and 69 essentially repeat the instructions in verse 65.
It is possible that this separation of the righteous from the wicked refers to the last days when Satan will be bound. The millennial state will be one where the wicked have been cast off, leaving only the righteous. This is an allegory covering the overall task of the Lord of the vineyard, so looking to that future date fits with the great scope of the allegory.