To play Devil's advocate here; if you punish someone for mistakes made when they were younger, and that they have shown at least some improvement since then, there is little incentive to try and be a better person.
That exact point was being debated and discussed on NPR today as I was driving home from work. The guest (didn't catch his name) did a capable job of dismantling that defense for Northam, in great part with Northam's own words, which show that his improvement is based on changing norms of what is socially acceptable rather than any true enlightenment. Every time his apology veered close to the mark, he'd hare off on some cringe-worthy tangent.
One example (and I'm paraphrasing here) was that Northam admitted that his experience of life was different from what black people experienced and found hurtful -- so far so good -- and then he HAD to say it: and vice versa. It's the "vice versa" that plunges Northam right back into the false equivalencies of racism. No, this isn't about two sides getting equally hurt by our different perspectives. It's about ONE side always getting crap thrown at them. It's always about ONE side having to make allowances for racism.
I can understand why a lot of black people are tired of endless "dialogues" about racism and would like there to see some real consequences for a change.
Honestly, I don't know what the statute of limitations should be for an act of such casual, arrogant racism. It's not just the blackface, it's the KKK figure in that same frame. That adds a level of violence and menace that goes beyond the distasteful mockery of blackface. Would people feel less forgiving of Northam if there had been a noose in the background? Would that be a bridge too far, where this is not?
How do you show enough contrition? What would that look like?