1. The crime happened. All parties agreed that it happened. A child was raped, Polanski did it.
2. Polanski fled the country while awaiting sentencing. So, the legal, correct, & useful thing to do is to bring him back to face a judge, complete the sentencing phase.
3. Why? To reassure the nation that even talented, possibly wealthy, & well connected individuals cannot get away with raping a child.
And if there are other factors to consider, such as the original judge in the case changing his mind about a plea arrangement, those can be discussed during Polanski's new sentencing hearing.
A horrible thing happened, a crime was committed (a rape) within a country that has a legal code that punishes for such activities. To exclude Polanski from punishment would not be fair for everyone else - including other criminals who were punished for raping children, and rest of society that follows the law.