Powercreep plays the same role in games like Elvenar as monetary inflation does in economics. Too much is bad, but you basically have to have some for things to function properly. Have a thought experiment - what if we had negative powercreep? I.e. every new event we get buildings that are strictly worse than the ones before that? Well, you know what will happen - no one will play new events as you'd get all you need and sit on that forever as it will always be better. It's slightly different while one is progressing through the chapters as worse building in a higher chapter may still be worth it, but once you're approaching late/end game, your event participation will stop. That's not a great setup. No powercreep works the same way. If new buildings, however, are ever slightly better than then ones before, you're always tempted to participate in the events as your old buildings ever so slightly getting obsolete, even if you're at the end game.This might be an unpopular point, but I really want to raise the question if Inno should not stop to offer "power" items (like the phoenix or the bears) as main rewards. This just leads to a power creep destroying the game balance in the long run (which is anyhow not a strong point of Inno).
If the grand prizes are mainly cosmetic (maybe new skins for existing buildings) it wouldnt pose a problem if people win "too many"..