Its obvious that hearthlings skills become rusty when they are not used!
Make credos lose levels over time, to get the levels back, you would have to do some quests related to that credo.
Would make being jack of all trades more difficult, alts would not help, because you have to do quests!
You would have to specialize in something, or then just accept the fact that you can not do everything perfectly. It's not like you would need credos to do the things. Having credos is just a nice benefit.
Pre-req credos would not decay if you have levels on some other credo that requires them, you first would lose the most "advanced" credos, after that the basic ones as well.