The assumption of this verse is not that the call to repentance will be effective, but rather that it likely will not. Jacob's presumption appears to be that they will continue in their path, and he is asking if they are fully cognizant of the consequences of their actions. Those actions will lead them to a condemnation before God.
Sociological: It was noted in the discussion of Jacob's first discourse in the book of Jacob that he appears to have to legal power to compel the people. That separation from the seat of power is even more apparent here. To fully comprehend the significance of the pleading Jacob before the congregation, we need to rehearse the nature of many ancient societies.
For the ancient world, the realm of religion and politics were not that far separate. The Jerusalem inheritance of the Lehites would have been a political king, but a king serving under at least the presumption of the support of religion through the anointing ceremony. In addition, much of the other political offices would have been occupied by priests. In the subsequent years after the fall of the first temple. the priestly class exerted even more political power.
In Mesoamerica, rulership was also presumed to be sanctioned by the gods, and ruling families were also likely to supply priests in the smaller city states, though this situation changes with the larger state-like Aztec empire. Nevertheless, the ability to rule even in the late Aztec empire was dependent upon tracing legitimacy to a line sanctioned by the gods.
In societies with such strong religious underpinnings of social structure, the threat of religious sanction was extreme. The Jerusalem example from the time of Christ indicates the power of a priestly class to effect a death sentence, even when they themselves could not pronounce it. So in other ancient cultures, the variety of religious sanctions could easily range from death or exile to milder forms of control. In a society as closely intertwined with religion as were most ancient cultures, the violation of a rule of god was a violation of the rule and sanctity of the community itself. For many cultures, violating a rule of god risked the wrath of that god, a condition which could not be tolerated.
We have in Jacob a situation where a number of the people are violating a command of god, and Jacob exercises no direct authority over them. He does not threaten exile/excommunication, which would be a typical way of handling a major threat to the social organization. He does not declare them under sanction of the ruling body of the religion. Jacob pleads.
We may read into this situation a couple of principles. First, the concept of the personalization of religion was instituted very early in Nephite society, such that the moral responsibility for religious orthodoxy was individual rather than social. For the modern world this may seem all too obvious, but it is a significant departure form most smaller ancient cultures. Perhaps this is the heritage of Jerusalem showing in the way the gospel was preached, for this is a religious principle typically advanced for ancient communities, but was present to a fairly great degree in the practices of Israel. Biblical Israel was often decried by the prophets for her attraction to the high places, a reference to foreign worship. Thus the nature of religion among the Nephites was significantly different from that of surrounding communities not only in the doctrines, but in the individual versus community emphasis on the way religion was implemented.
We may next infer that Jacob has no political/religious power. While he is able to decry the actions of his population, there is nothing he can do about it directly. This suggests a greater division between religion and political power than would have existed when both Nephi and Jacob were together. In the time period after Nephi's death, Jacob continues to appear marginalized in his direct authority over his people, even when (as is the case in this discourse) when he is declaring prophetic authority for his words.
With the hints from the earlier discourse, and the continuous nature of what Jacob clearly sees as a major apostasy from the right way, we may see a picture of a community under rapid social transformation in the years after Nephi's death, with a consequence being the alteration of the lines of political and religious power. Most clearly, we will see that Jacob's lineage will be increasingly marginalized, and when we emerge from the small plate tradition to rejoin the large plates, the nature of politics and religion will perforce be of a different character than pictured in the small plate tradition.
Certainly the good news is that the schism between politics and religion becomes healed, with a return to the king as prime religious figure (such as King Benjamin), but the lineage paths for the description of power will have altered such that Jacob's lineage is no longer part of the priestly tradition, with that focus returning to the regnal line.
It is not as clear how the social fabric will fare through this transition, but if the inability of Jacob to effect a change at this early stage is a telling model, then we may expect that the continued affluence of these Nephites with their external sources of wealth will continue to alter the expectations of the social structure. By the time of Benjamin, we will likely be dealing with a thoroughly Mesoamericanized physical culture, with a unique religion that remains attached to the central government. Since Benjamin is also involved in the transition to Zarahemla, we will also see that the Zarahemla manifestation of religion will probably represent a renewal of the earlier religion.