The changes in society won't affect people's ignorant views so much. As long as there is something to reinforce what they think, they'll keep thinking it. It doesn't matter how many people break a stereotype, they're just considered "excepts to the rule". As for Imus and people like him, they know exactly what they're saying. He knows that what he says makes people angry, that's why he does it, and that's why there are people who like him.
As for Sharpton and Jackson, a lot of people know that they've done dirt. But when stuff regarding prejudice against Blacks comes up and they have a chance to be on TV, they'll jump on it. When people who are also against the prejudice want to be heard, they'll call those two.
Sharpton isn't a good representative for racial equality, but I think saying that he makes David Duke look like Papa Smurf is too much.