I got to admit, I don't really like the sound of some of the members acting like this is really terrible news. Retiring from roles happens, especially since the show's 45 years old and she was there since the beginning. It's a little disappointing, but she's been with the show around as long as Carol. It's unprecedented for a kid's program to last this long with absolutely no signs of slowing down, and the fact we have as many cast members still with the show as long as it is still running is amazing itself.
That said, I don't get the "as long as Prairie Dawn's not replaced by Abby" sentiment. Abby's a completely different character with a completely different schtick. Her job was not to replace characters, but to add a marketable female presence to the show. And yes, it came off a bit forced. But, frankly, so wasn't Zoe. Even down to the making her Orange to compliment Elmo bit. I'd say that Abby is gunning to replace Zoe, with more Elmo and Abby focused episodes. Elmo and Zoe are a much better pair with better chemistry. I ship Zoe X Elmo even though I am incredibly opposed to pairings.
Seriously. Try to watch Elmo in Grouchland and not see there's something between the two. Like in a little kid way.