When I first started playing, I thought that "reach rank 20 and earn that season's card back" meant that each season had a new card added to the game, and if you reached rank 20 at the end of the season that card would be added to your collection. The more I think about it I like the idea though. Make it so you can craft it with dust if you happened to miss it or didn't start the game until later, but everyone who reaches rank 20 gets the card.