@p3temangus We don't know whether the popularity of Pioneer will last. They also could add cards for Pioneer to Arena some day. I don't think this would be impossible if Hasbro determines it would be profitable enough to do so.
"The one and only" could be referring to many things, or it could be a pointless phrase thrown in to catch attention. It could also mean that "the one and only" card that matters in Arena is the one pictured in the left hand side.