UPDATE: We will be able to add level 60 Relic Hunts possibly tomorrow. The coin reward amount and Golden Chest rates will double from the lv 50 bosses, but the golden chest content has to remain the same.
Why do the items in the chests have to stay the same? For how hard rh60 is it seems we should be able to get rare items as a reward. They are really tough.
The gold/silver chest content being the same is a limitation of the current implement of the feature. We may look into break this limitation in future updates.