Margaret has been missing for almost 50 years. Without DNA to connect a potential perpetrator, investigators are restricted somewhat to older methods.
The theory of Margaret being the victim of a serial rapist/pedophile/killer is one with merit. A study of known serial offenders might turn up something.
One example (out of many) who comes to mind is Robert Bowman, currently serving a life sentence on Ohio for the abduction and murder of Eileen Adams in 1968.
I haven't seen it anywhere else, but there's a podcast about Eileen that states that Bowman died in prison.