I think the police may be halfway content if they can at least get the person at the top of their list put away for a long time, even if it is for historical charges. Their primary POI may then sit out the rest of his years in prison. I truly think that they know what has happened to William, but are unable to appease a prosecutor due to the incomplete nature of the evidence.
I recall John Chardon being the obvious-but-unsaid primary suspect in Novy's disappearance, and they searched and searched for Novy - still haven't found her to this day. So they changed tack, and were able to successfully prosecute Chardon for several child sex offences that had been lurking in the background for some years ... a good 15 years or so, iirc.
Chardon was locked away, which hypothetically may have kept his (Novy's) own two young children safe, and definitely will have kept their friends safer - as his now-adult children's (from a previous marriage) friends were his previous victims, years ago.
While locked away, police were finally able to pull the case together for Novy's murder, in a manner that suits the prosecutor. They have been able to charge Chardon with Novy's murder - 3 years after Novy's disappearance.