One problem is that this has follow-on effects of reducing sentences for other crimes. For example, murder would be 20 years because that is the cap. What do you make rape? You don't want to make it 20 years since then that incentivizes the perpetrator to always kill the victim. Given the above, you are looking at 10-15 years at most for rapists. Then you need to differentiate the serial rapist (see Larry Nasser) from the non-serial rapist. All said, you are probably looking at sentences of 5-10 years for even the most heinous single rape conviction.