The court said that, as a result of the prison attack, Poi would likely need lifelong care.
"Despite his release on parole into a nursing facility, Mr Poi's family contend the original sentence was too severe," the judgment said.
But the three Court of Appeal judges who heard the appeal ruled the "deeply unfortunate post-sentencing event" of the prison attack was not grounds for a reduction in Poi's sentence.
Poi's own attack on his victim resulted from an argument and fight over cigarettes claimed to be stolen by a friend of the victim.
The man was knocked to the ground and punched, kicked and stomped by Poi and his flatmate for about half an hour.
Poi hit the back of the man's head repeatedly with a bottle and when the pair were satisfied the victim would not call police they made him shower to wash off blood from the attack before letting him go.
The man was left with seven lacerations to his scalp, a fractured rib, a broken tooth and other minor injuries.
Poi's lawyer, Tony Snell, described what had happened as a tragic situation.
Mr Snell said after the successful parole application, it was not surprising the Court of Appeal had dismissed the appeal against Poi's sentence, although the outcome was "academic" given his client's comatose state and need for ongoing medical treatment.
Police said yesterday their inquiries into the assault on Poi were ongoing.
Mr Snell said given the attack on his client had happened in a prison wing, there were a limited number of suspects.
"What it will take is for somebody to break ranks and talk before they get a prosecution, and whether that will happen, I don't know," he said.