I've noticed a decline in activity of those performers in recent years due to Muppet projects. Steve was only available for one skit this year, and it was an Ernie song. Eric's characters (mostly Grover) made a lot of insert segments, but significantly less as far as the main stories went. I've noticed Joey, Marty, Carol, Kevin, Fran, and Leslie (and some others) basically holding up with their characters this year... I wonder what they were busy with.
I hope that those guys make it back to Sesame Street this next season.. they're the characters we want to see most.
And yes, I hope you're right about when SS is done, they film some new special stuff like new Virals and videos from their own site.
In the mean time, I don't understand why Disney just doesn't take some of the Muppets.com stuff and release it to youtube, like they did a couple Halloweens ago.