The issue is that there's too many goals people reach for. Below is a thorough explanation of what the problem is and how it can be fixed.
In the example of starcraft 2, other than the goal of having fun, the only other goal is to get to a higher rank. In that case, everyone went for the same thing, and all was well.
However, in Altei, there's 2 goals. One goal is to have a very high RP, to display how good you are as a player, much like the ranking system in starcraft 2. The 2nd goal however, is to obtain more cards. Obtaining more cards involves either 1, getting more weekly gran, or 2, winning treasure duels. Due to the difference of these 2 goals, there will be a divided amount of people doing perusing different things, thus, the entire idea of "keeping the game fair" by RP split is defeated.
This is more of a fallacy of game design than anything else, and it would require massive amounts of redesign to fix. If somehow, the RP and the amount of cards you get would be tied together, for example, maybe the RP constitutes how many cards you get at the end of the week, or maybe the RP determines the amount of treasure duels you have, then RP itself would be the sole goal for Alteil. But, that would raise a much bigger issue, and that would be that the better a player, the better he would be, and the worse the player, the less likely the player will get better. It would create a separation of the good and bad, much like how there's separations of the rich and poor, the rich will get richer, and the poor will stay poor, or look even worse in comparison.
This is where the RP arena comes into play. If the above system was implemented, it would create a horrible system. However, if RP arenas were established, it would once again make it fair. The rich will only be compared with the rich, and the poor will be compared with the poor. Everyone would have the same goals of having the highest RP, and all would be well.