Crozier's lawyer Marcus Zintl asked that he be sentenced on the basis that there "was one single blow".
Justice Nation said the aggravating factor was that it involved "extreme violence without provocation".
Crozier was "a big strong man" and the medical evidence suggested Thompson was injured by Crozier wielding a bat with "substantial force" and inflicting a severe head injury on one of the thickest parts of the skull.
Thompson and his companions left Crozier's house immediately after the attack, not realising the significance of what had happened until the middle of the next day, when an unresponsive Thompson was rushed to hospital.
"He was fortunate to survive," Justice Nation said.
Crozier had continued to deny his culpability and was unapologetic. He was assessed as a "low risk" of reoffending.
The Justice said Crozier's positive work and community history - supported by 20 character references - made his action that night inexplicable.
The references showed he was "capable of being kind" and being a role model, "so how could you have shown so much violent aggression to someone who had never done you any harm?"
Crozier had continued to deflect blame, telling a probation officer his victim was injured when his face had "struck a post".
Given the circumstances and evidence, Justice Nation said that was "inconceivable".
His history of assaults in the 1990s and a 2012 conviction for insulting language also showed Crozier was "quite capable of real violence".
At the time of the attack, Crozier was at home celebrating the birth of his son when Thompson and two friends arrived to join the celebration.
Thompson was not present in court but his victim impact statement was read aloud.
He had a family relationship with Crozier and had previously counted him as a friend.
Thompson said his life had irrevocably changed following "extensive brain damage", including "visually grotesque" physical scars from months of intensive care to save his life.
He described the battle to get back on his feet - taking five months just to take one step in rehabilitation. Before this he had been a fit and healthy, financially independent builder who worked hard.
Now he could not do simple physical tasks alone, would need treatment for years, and felt he was an imposition on his family.
He had lost feeling down one side of his body, was visually impaired and had short-term memory loss. He could no longer live independently, work, drive or fly; it was degrading and embarrassing.
"It stopped my life in its tracks, even at my age. My hand is like a claw, it's useless. I have to eat like a Neanderthal ... because of a vicious assault on me from Denis.
"I feel like I'm a burden on people. My self-esteem has gone. I don't know who I am any more."
Mr Thompson also described "destructive thoughts of revenge".
"I feel totally betrayed ... we were good friends. I don't want to know him."
