Just a suggestion. Then why not add more soulbound rewards, even merits to the rewards and remove almost (if not all) sellable rewards? At least do this in Bronze, Silver, and Gold where the one's in the leaderboards are not actually Bronze, Silver, Gold players/accounts but extra accounts from those in Champion league?

That way, even if you give, say 10m cards, it doesn't matter since it isn't sellable. It only matters in terms of making your cards/team stronger for when you try in higher league such as Diamond and higher.

So, "technically" it makes it to not worth to farm in the lower leagues and try to actually farm and compete in higher league's leaderboards where most players there are competitive players.