But there are some things that even post-singularity medicine can't fix. Inside, Gu is still an old man, and still a royal asshole. It was an act of some courage for Vinge to make his protagonist someone who is really a mean, vicious person. He's not just a cranky old fart, though he is that. Nor is his personal unpleasantness just a hard shell under which lurks a soft, kind center. He is, at least in the beginning of the novel, selfish, arrogant, manipulative and emotionally abusive to nearly everyone around him. When his 13-year old granddaughter Miri tries to help him adapt to the mystifying technology of the new age, he responds by telling her:
"You've spent your whole life playing video games, convincing yourself and your friends that you're worth something, that you're some kind of beautiful thing. I'll bet your parents are even foolish enough to tell you how clever you are. But it's not a pretty thing to be bossy when you're a fat, brainless brat."
Note that this is not just some crusty thing he says before he's eventually nice to her. Rather, he's spent some time getting to know her so he'd know exactly what to say that would hurt. He does the same thing with several other characters. Not surprisingly, he begins to change over the course of the book, otherwise it would just be too unpleasant to read. But he is a sympathetic character in spite of the fact that he's a jerk. More importantly, with every reaction he makes, and with every interaction he has with people, you can feel the history of his 75 years getting in the way.
Vinge has an advantage here, in that Gu is surrounded by people he has interacted with his whole life, while Scalzi has shot Perry off into far space, literally dead to the world he knew. But even separated from everything he'd ever known, Perry should have had a history. Once he mentioned his former job, but I don't even remember what it was. It certainly played no part at all in the story, which seems a waste of narrative energy. The point was supposed to be that anything a person could have experienced on Earth would be no preparation at all for what one experienced fighting aliens for the CDF, but even the simplest life is full of meaning that shapes your view of the world.
Gu's history is not just background color; it's a trap he's trying to escape. Through some kind of side effect of his treatment, he was restored to full mental health without the one talent that made him famous, the ability to make words sing in poetry. In exchange, he has gained talents he never had before, especially a sudden ability to work with and create technology, a skill that is far more beneficial to him in the modern world. But it's not easy for him to start over, to become another person. He still wants to be the great artist he was, meaning he may be willing to risk selling his soul to various online devils in order to get his old talent back.
In other words, he is that strange mix of cynical and naive that old people can become. I don't know what would become of him if he were sent off to fight aliens with the CDF. But I suspect he would have higher priorities that kicking ass and getting laid. He might get killed faster than a 75-year old who acted 22. But he would certainly be more interesting to read until the aliens sucked his brains out.