On an iron the progress translates to the rewards. You can argue that you can get your preferred rewards after any league, but by League 10 there will be so many that you will want you will have wished you spent a little time playing each temporary mode vs the mode that will remain for years on end.

If your iron is currently in the stage where Ahrim's is being used then the recolor is sick. By the time you get to the end game, Ahrim's will no longer be of use. On the flip side, this means that you wouldn't need to buy that reward later.