The last chapter of KHR really pissed me off because Amano-sensei basically undid all of Tsuna’s character development.
How so? I actually think she did a good job bringing the manga full circle by contrasting Tsuna’s initial position as an insecure loner with his final position as a maturing, developing young boss surrounded by the people he loves and protects. He even says that “after all, I am not alone anymore”. And that, I think, is the most important aspect of his development.
Also, keep in mind that while he does reject the position of boss even at the end, he will become boss ten years later. At the stage he is right now (as in, the end of the manga), he’s not at a fully developed point yet.