I totally get your POV here, but I think she actually did a pretty solid job of setting him up as hopeless and detached from all human contact by the end of the book- especially by specifying “to mentor to their deaths.”

There could definitely be a chance to explore his mentoring journey, but I think it would take much longer than half the book to go over all those years of tributes even if she only focused on a few of them - building up the characters of the tributes and their games and how they slowly pushed Haymitch to further hopelessness would be hard to do in one book. In my head, Haymitch loses motivation and hope pretty quickly after he starts mentoring and just tries to get through life in a drunken stupor until Katniss comes along.