Arsenio is back, so there’s one non-white host. There’ve also been failed shows over the last couple decades by non-white guys like Magic Johnson, Keenan Ivory Wayans, and George Lopez.
Perhaps one issue is that where the white hosts more or less tend to go for a more mainstream, broad appeal, some might say “bland” guest list, where someone like Arsenio tends for a different group of guests. Which is great, there should be a forum for the sort of guests the Lenos and Lettermans (Lettermen?) would never touch, but it’s just not going to draw from as big of pool of potential audience, so you’re really not going to be able to have more than one show of that type going at a time without the shows stealing ratings from one another and putting each other out of business.
I certainly agree with your a and c. I don’t know about b but won’t argue with you. As for your d, I wonder how many folks watching Joan changed the channel at 11:30 PM eastern to Carson. Though what with c, there wasn’t much reason to watch Joan anyway.