Very interesting. I'm looking at modifying for Bronze level. It's undetermined whether the rewards chest conditions will be updated this season. They have announced that it will be, but this is the third time they have said that, so who knows.
Your statement about not needing to maintain power to get additional chests only holds if the they backtrack on this once again. Then, it will remain highest rank achieved during the season.
Otherwise, it will be rank you are in at end of season, and you will have the power then.