For alot of old players the game no longer became challenging to them, its pretty much dull and boring. To make the game interesting to them you need to make a way if you make it hard on high lvl players but keep it easy to low lvl players so they can participate in the event and not quit the game because they cant do it.

So most other games use a scaling system, which depends on a certain criteria which in this case would be the Idols count.

Make the event quests depend on the idols you have, the more idols you have they stronger the monsters are, i mean come on, reach floor 200 only using none human crusaders? i can do that using only a ww....

In return make the quests more rewarding, maybe a since the monsters are stronger that means more gold which means more idol counts or something.