Conviction rates have nothing to do with the severity of the crime. Convictions are based on the quality of the evidence - guilt proven beyond a reasonable doubt.

Criminal statutes do not use the word "serious." Classifying a crime as a violent felony, the most severe classification, indicates that it is taken seriously.

You asked for evidence, not proof. It cannot be proven since the appropriateness of sentence lengths is a matter of subjective judgment.

I worked inside a maximum security prison for 33 years. Rapists were serving sentences equivalent to those convicted of aggravated assault. Whether that sentence is severe enough involves subjective judgment. If the law were to prescribe longer sentences for rapists I would not object. However, I am not uncomfortable with the current sentencing guidelines.