While I can see why you're making that argument, Chilldoubt, it seems to me to be rather a non-sequitur. The Oscars selection process can still be biased against a particular race in general, even if the occasional winner makes it through. Put it another way, do you think that America was a country that had zero problems with racism in 1963, when Sidney Poitier won the Best Actor gong? Clearly it did, since the civil strife at the time was pretty severe.
I've argued earlier that I don't think this is necessary an issue of racism at the selection process (at least, not overall), but rather a bottom-up issue of black actors not being given enough of the roles they would need to attract attention for Oscar consideration. All the same, the fact that black actors have won awards in the recent past doesn't exonerate the Academy of racism either. It's a complex issue, and I'm not sure the boycott is appropriate, but Rampling's response is not appropriate either.