I think that Sinclair believes in the motivating force that Jurgis represents. Sinclair does feel that Jurgis' narrative and anyone who has absorbed and understood it will leave the novel invariably being an agent of change, a force of positive action in the world. It is difficult to see that Sinclair not see the people who are inspired by the book as heroes in "an army of construction," as Keller would say. Yet, I think that Sinclair also realizes that there will be formidable opposition to this army, and he does not back away from this reality. In the social context of the novel's publication, Sinclair understood that one of the main tenets of Progressivism was the need to fight, regardless of context, and to embody something transcendent in a world of contingency. In this, the ending of the novel is one in which Sinclair understands the massive challenges for social change in front of America, but also grasps that individuals can make and create lasting change, if there is solidarity embraced and if one finds themselves committed to it. In this, there is positive affirmation.
Where I think that Sinclair might have possessed some level of myopia is in not understanding that capitalism, as an economic structure, is pliable. Sinclair's depiction of capitalism is one that is fixed, rigid in its application. Sinclair did not fully grasp that collective bargaining and unionization can be evident, but still not fully rectify the wage imbalance present. It can simply make it a bit more tolerable and this would be sufficient for many. Sinclair did not fully grasp that union leadership can be corrupt, and not always zealously fight for the cause. Sinclair did not fully understand that capitalism is strong enough to endure changes, and in this, its paradigm will still be accepted by individuals. This enables it to remain and along with wealth imbalance and inequity as a result. I think that this might be where Sinclair's hope and optimism that comes out of the end of the novel could be tested and placed under some level of stress.