Symbolic: Understanding this verse depends upon understanding the image of the "loftiness" of the vineyard. It is tempting to correlate the term "loftiness" here with the way that word is used to describe pride. While pride might be accounted as a problem with humanity it does not fit precisely with the image in this verse.
The description of what is meant by the loftiness of the vineyard is given in the verse itself: "And because the branches have overcome the roots thereof, behold they grew faster than the strength of the roots, taking strength unto themselves."
The servant's response agrees with the master that the efforts of the Lord of the vineyard have been sufficient. The fault lies in the loftiness, and that loftiness is directly defined as the overgrowth of the grafted branches. In an ironic way, the efforts of the Lord have been almost too good, for the grafted branches have grown faster than the strength of the root allows. The fault lies not in the Lord, but in the branches.
The allegory therefore squarely places upon mankind the responsibility for the failure of the gospel (up to this point). This is an important theological point, for it is important to be clear that the temporary failures of the full benefit of the covenant with God are never due to his actions but always to our own.