They don't need to decrease anything, we already don't have enough stuff.
What I don't like overall is those who already have a lot of materials stored, would never have to buy gems to keep participating in the buff. Those who don't have anything are the ones that will pay. Which is common sense, but something about that doesn't sit right.
Those who already have lots of rewards risk nothing, those that don't risk everything.
Also there's the whole idea of having to decide if you want to get the highest buff in the week or open chests. I'm not rolling in gems and I already know my answer to that dilemma.
Of course i'm not complaining, just pointing things out the way I see them.