Richard was one of a kind, and he had an amazing and unique voice. Fraggle Rock Animated's Junior Gorg proved that (difference here, at least these guys are TRYING to sound like Richard).To be fair, I'm not sure anyone could completely match Richard's enthusiasm!
David has been recast in a LOT of Richard's roles, going far back as the Cosby Show where he performed Sweetums while Richard was a sandwich.
When it comes down to a performance, there's several things to consider... personality should always come first, availability a close section. Sounding the same is important, sure, but not as the others. That's why certain other characters bounce back and forth from performer to performer and some never even get a major recast. I remember Dr. Teeth going through several performers, and only for one line at a time in projects. A character is a very personal thing, and sometimes you luck out and find someone who can do both a voice and a personality...
I am grateful for recasting... I believe we should have new characters, but keep the old as best we can. When someone leaves or passes, those characters cause a gap... and a lot of characters get created and lost to fill that out. Sesame Street to be sure... that's one of the reasons there's so much Elmo...