The vineyard fails continually because of "the loftiness of the vineyard." (5: 48.) That is, the pride and arrogance of Israel itself is the cause of continual failure. They run faster then they are able, reaching what they cannot attain, claiming to have what they do not have, and relying on their conceit rather than the Lord. As a result, the branches overcome the roots. They grew faster than could be accommodated, and took strength to themselves, which always defeats fruit production. (Id.)
The Lord's exasperation with this complete failure results in the announcement that He was going to return to the vineyard, "hew down the trees" and then "cast them into the fire" so they no longer cumbered the land. (5: 49.) It was time to return and destroy everything. Or, to use a phrase from Malachi, to "smite the whole earth with a curse." (Mal. 4: 6.) We see in this that the Lord does actually consider smiting the entire earth. The allegory reveals it. It is indeed possible for the Lord to consider that as an option.
The only way to prevent it is for the "vineyard" to again bring forth fruit worth laying up against the season of the harvest. It failed. There was a universal apostasy. The Lord announced it was His intention to destroy all the people of the earth. (5: 49.) But it was the "servant" who pleaded for the Lord to "spare it a little longer." (Id.) In Zenos' allegory, the Lord is the one wanting to destroy the vineyard. When He was in His mortal ministry, the Lord reversed these roles. He had the angels wanting to destroy, and the Lord being patient. (See, Matt. 13: 28-30.) In both, the judgment is postponed until something worthy of preserving can be brought into the harvest. The Lord agrees to spare the vineyard despite the universal failure to bring about "fruit" because it "grieveth" Him to see such a loss, so great a waste. (5: 51.)
Now all of this is about history. It has already happened. Zenos wrote in the unified kingdom, before the division into the Northern Kingdom, or Kingdom of Israel, and the Southern Kingdom, or Kingdom of Judah. He wrote before Isaiah, and before Jeremiah. His prophecy became a benchmark from which other, later prophets would draw in fashioning their own prophecies.
Using these allegorical themes and images (tree, branches, transplanting, grafting, laboring, gathering, burning, trimming, pruning, etc.), we can see what happened historically with the scattering of Israel. Now, however, we have reached a point in the allegory where the events are either current or future. They are underway. This part of the allegory relates to us. It is meant to warn us about the time we live.
We think we've gotten the benefit of the Lord's hand in the effort now underway. However, there is nothing going on at this time in the vineyard that should make us think we can relax. There is more pruning, gathering and yet more labor, before we yield fruit.
As we continue from this point forward, we must pay more attention. It is a blueprint for how the Lord is dealing with us. We should take every opportunity to consider how the prophecy may be intended to warn us against our own "loftiness" and ultimate failure.