Well I didn't consider Raj in that light until you brought it up a long time ago, and I tend to agree with you here.
The thing is, why not introduce a new Black character. There are many BM nerds. engineers, etc that could find a place on the show. IT'D BE NICE IF HE HAD A WHITE GIRLFRIEND, too.
And where are the Asians? Time to update the show.
I haven't seen The Big Bang Theory. And I do notice that there are no black nerds on the show. I remember black actor Phillip Akin played a wheelchair bound scientist who assists actor Jared Martin in The War Of The Worlds tv series(I hated that he was confined to a wheelchair). I remember Asian men and women were the designated tech specialists on a lot of shows(example; Star Trek's Lt. Sulu went from a helmsman to astro physicist to starship commander, Star Trek: Voyager's ensign Kim was a helmsman and an accomplished musician and a virgin, The Goonie's Data was an amateur inventor whose contraptions often malfunction). The show Scorpion has an Asian woman in the cast(I assume that since the Scorpion Group is a growing group, a black male may be added to the cast eventually...or not) Hawaii 5-0 has Masi Oka(and he sometimes has a white girlfriend and Chai McBride has joined the 5-0 team as an older member who will remain single). For the major television networks, this is about achieving the "Friends" effect. The establishment of not only a fan base, but a community. This, of course, based on the writer/creator of these shows and how it is sold to studios who produce them and the networks that pick them up. This is why for a show to promote diversity, it should be shown on independent studios and networks.