Yes Robbie hasn't even been gone a year, he came back for Martin and Sonia's wedding (which he didn't end up seeing) in May (I think it was). It would be ridiculous to bring him back with another actor playing him.
I do agree though that some characters who come back played by a different actor sometimes work. Of course Charlie Brooks as Janine, but she didn't have much of a character when she was in it the first time. Todd Carty playing Mark was good, although I wasn't really old enough to remember the first Mark. I don't think they have changed the actors of characters much in EE, except when somebody goes out of show and are brought back, or when child actors finish and they bring somebody else in to carry the actor into adulthood.