I am utterly puzzled as to how anyone can feel that way. To my way of thinking, it was highly significant and affirms all of what we have been discussing in these pages: that the story is one of renewal and redemption. That by confessing and purging one's soul of guilt, by adopting wholesome and regenerating Christianity, by society imposing retributive justice, by mitigation because of extenuation both in life and in the times when he was a suspect, that his confession saved society much litigation and investigatory costs (the epilogue says no actual evidence was found and that it was Rascal's detailed confession that enabled the police to find the evidence used against him in court), the fact that he did not take flight, that he had given money to save an impoverished father from bankruptcy when his son died, the fact that Rascal saved the lives of two young children and by jeopardizing his own well being in making that sacrifice, and that he genuinely was repentant ~ all these things were factors in why society imposed a relatively mild sentence.
From my past readings of classical Russian literature, I would have thought Ras would have been condemned to the gallows. I had no expectation that mitigation would be granted and that he would have been enabled to live out his life in a serene manner with Sonia. The narrative tells us that she was accepted as a guardian angel by the prisoners whim she helped in writing letters, providing goodies, and comforting them in their grief and isolation. Dostoyevsky concludes the book by telling us that both Ras and Sonia were renewed in life and lived happily thereafter. Note how there was no talk of nihilism, of reform, of Westernizing influences of any kind. Instead, Christianity was their saving grace. This message, no doubt, is what the author was trying to convey for his readers.