I believe arena rewards are linked to overall winrate and plays.
So if people play an arena too much or win too much, the rewards fall, and if the opposite happens, the rewards rise. But then, now everyone is farming bronze or silver and they aren't going down or up: I believe this has to do with an interaction between all arenas' rewards. If one arena has its reward lowered, the others rise(although at this point it's just a theory of mine).
Perhaps some people are taking advantage of the NaNs in gold and platinum? They have a good reason since upped special spins are there, too. But then again, I think it's some kind of super-farming; it's far too much to have such a low reward considering what platinum is and that the decks haven't really gotten a downfall in their win:loss ratio.