Based on watching the first and last graphs alongside the ones you mention, my take is that BAU is somewhere between flat and slowly increasing. From there, when powerdowns rise, the percentage declines, and when big outside powerups happen, the percentage rises.

IMO, the bottom line to explain the long flat curve is that the big powerdowns have outweighed big powerups for about the last quarter.

I could be wrong, but I don't think the curve tells us much about the clubs. I think the daily rewards are too small (as a percentage) to show up here.