I'm not sure it's necessary, but I certainly think that would be a superior solution to having a list of offences, or this dual approach involving maximum penalty and prosecutorial discretion.
You could say that if denunciation and deterrence are uppermost in the court's mind, then the court should go against a conditional sentence. But the simple point I want to make is that there are going to be cases, even for the most serious offences, where the court will recognize relevant mitigating factors and want to impose a non-custody conditional sentence. I think taking that away from the courts is a mistake.