That's not even close to what social justice is about; the issue isn't what anyone's ancestors did, it's about the lingering effects of their prejudice that are still embedded into our systems and culture thus still affecting people today. No black person alive today was a slave, but they're still suffering the cultural prejudice that came from the slave era, things like mandatory minimum sentences that are much harsher on drugs common to the poor and black than the white an well off is one of many such examples. Bigotry is alive and well even today and minorities still suffer constantly from it; that's what social justice is about, recognizing that things like slavery may no longer exist, but it's effects still do and so it's still a problem that needs addressed.

What about those who seem quite happy about such recent incidents as students literally burning 'colonial' artwork or asking for statues to be removed? That seems pretty extreme.

No one's saying the legal system is fine (though that said, it should be pointed out that it's biased in more ways than one and on more than race), but there's definitely a bunch of people trying to tar everyone with the same brush, and thinking that certain things are the only differentiators.

Asking for statutes to be removed is extreme? Burning things in protest is extreme? I think we use the word extreme differently, seems pretty tame.