What kind of career path might be possible for a professor following a plagiarism scandal?
Past a certain point in their career and education, I think it's hard for somebody to ever clean off the stain of serious, prolonged, and deliberate misbehavior.  The more that a person has built up authority and trust on a false foundation, the harder it is to ever trust them with regards to that subject again.

When a young student engages in academic misbehavior, we treat it as a teaching moment.  When an undergraduate does it, we try to put fear into them and rehabilitate them.  When a graduate student does it, we try to figure out whether it is worth salvaging.  When a professor does it, it pretty much destroys their academic career.

And to my mind, I think that's OK.  We're talking about serious and deliberate professional malpractice that undermines the basis of the whole endeavor.  That can waste millions of dollars or leave people injured or killed.  Would you ever again trust a doctor who deliberately injured their patients, or an engineer who deliberately mis-designed a bridge so that it might collapse?  I think that it is the same for an academic who engages in massive and systematic fraud.