The imagery of a grapevine often evokes fruitfulness, but the wood itself possesses a unique flaw. It is inherently unsuitable for building or crafting, serving no practical purpose for an artisan. This natural uselessness forms the basis of a sharp logical argument about a descent from wholeness to total destruction.
The primary approach among commentators is that this argument relies on a simple deduction. Even when a grapevine is completely whole and untouched, its wood cannot be used for any meaningful work or creation. From this reality comes a clear conclusion [רש״י, מצודת ציון]. If the wood is worthless in its perfect state, it is undoubtedly useless after it has been burned, dried out, and heavily charred [מצודת ציון, מצודת דוד].
Beneath the surface, this burned wood serves as a powerful reflection of the physical and spiritual condition of the Israelites [רד״ק]. In earlier times, when the nation was whole and living securely in their land, they stubbornly refused to repent. They ignored the repeated warnings of the prophets and dismissed the suffering brought upon them by their enemies.
This same logical deduction applies to their current reality. The fires of destruction have already consumed their surroundings, devastating most of the land and leaving only Jerusalem standing. Yet, rather than being awakened to change, the people have only hardened their hearts further. They have become so accustomed to their wrongdoing that forbidden actions now seem entirely permissible to them. Looking outward and seeing the prosperity of the surrounding nations, they merely wish to imitate their neighbors and worship foreign idols. Because the flames of ruin have already charred them and they remain completely indifferent to their tragic state, there is no hope left. Ultimately, the fire will consume them entirely.