We'll come back to David, but I want to add more to this Elmo discussion.
If Elmo had been around for so long it's funny, how in the "Street We Live On" the intention was for Grover to take Elmo to see the events that happen before he was born. However, most of the events took place after he was already on the show. In fact the only one he probably wouldn't have seen was Mr. Hooper and even then, the Brian Muehl Elmo was around.
A better idea would have been to take Elmo further back where he could have seen Matt Robinson, the orange Oscar, the original Baby Monster, etc. I know somebody once wrote a fan fiction like this, I'm not sure if that thread is still on this board or not.
Anyway, back to David. Northern Calloway who played him passed away in the early 90s, so his character had to be written out of the show.