If the C's had Telfair instead of Rondo, I think we would have seen Cassell take a much larger role once he joined the team.
And man was the league wide PG situation different back then or what? Instead of seemingly every playoff team having an MVP caliber point guard, the C's faced Bibby, Delonte West, Billups, and Fisher. With 31 year old Billups being the only All-Star, but not a guy who could single handedly beat you. Then if the East seeding shifted a little, the other PGs to worry about are Jameer Nelson, Jose Calderon/TJ Ford, Andre Miller, or an injured Gilbert Arenas/Antonio Daniels. I don't think the drop in defense the C's would get from 38 year old Cassell would be that big of a problem with the PG position being as weak as it was.
So ya, the C's probably could have still won it all.