Moko was left for four days suffering those injuries before the couple rang 111, saying he had fallen off a wood pile.
Canterbury University law professor Chris Gallavin is calling on the Law Commission to consider different degrees of murder for cases like this.
"New Zealand homicide laws are a complete mess, a total basketcase," he said.
"They need a review right from the ground up."
The Sensible Sentencing Trust also slammed the Crown's decision to not pursue murder charges, with founder Garth McVicar saying plea bargaining was becoming a problem in the justice system.
"This was Nia Glassie and the Kahui twins all rolled into one."
Dame Lesley Max, chief executive of child and family charity Great Potentials, said she was concerned the more serious charge of murder was dropped by the Crown, but understands why it was done.
"My sense is that in going to the manslaughter charge, they got a full confession and a very detailed confession, and they got a con-viction.
"A conviction is often much harder to get with a charge of murder."
Dame Lesley also said child welfare reforms should focus on preventing harm at home.
"Although it was not parents who inflicted this, nevertheless I think with an emphasis now on Government reforms, more on predictive risk, we might be getting a little closer, and I hope that would be the case."
Child Matters chief executive Anthea Simcock said the circumstances leading to Moko's abuse must not be allowed to recur.
"What can we do that we're not producing other adults who might end up doing such horrific things to children in the future."
She said there may have been people who had concerns about the situation and could have intervened.
"The message is always if you have any worries about the way you think a child is being treated, it might have ended up saving a child's life."
Rotorua Crown solicitor Amanda Gordon said she would not be commenting on the case.
