What's the penalty for Apple just ignoring this ruling, if any?
Afaik they cannot ignore a ruling. And they don't need 30% fees for maintaining IOS ecosystem - when you buy an apple phone for $1000, you pay Apple enough for it to maintain its AppStore. Also they earn money from AppStore advertising (which apps out of the millions should the small screen show); so maintaining AppStore should not be a problem. I don't know about the bad-actors part though.
They can, but it has a high probability of additional adverse consequences.