How might the tone of the novel be different if he hadn't escaped punishment?

Bad as he is, I don't think the reader wants Long John Silver to suffer a punishment would have most certainly been death by hanging. Had he been returned and punished, our adventure story would have ended on a sad note. Jim's emotions were always mixed, and he saw Long John differently than the others. He would have been devastated, and that would have dampened the entire reading experience.