I don't think we should consider cards from the Core Set as old as opposed to cards from the Adventure Packs being considered new.
There's a good chance all the AP and the Core Set were developed as a whole by the same design team during the same period. To us they may look new, but to the design team, there was probably some "plan" in bringing new features to the players (such as the Evasion mechanic we'll see later, Ally objective cards, etc.).