That's a decent argument, and sure they have diminishing returns. But with three dragons, your opponent will, on average, only get to play his cards once every eight turns.
Against the 'high cost' argument: Why not make a card costing 75 and make it instantly kill the opponent? High cost =/= nearly infinite power. Besides, it's quite easy to just pack 6 of these and 24 pillars. Maybe add the rare love weapon and the love shield.
