Honestly, why is the WC automatically mandatory? The LoN wasn't, the UN wasn't. The WC should be optional but the bonuses of the WC should be buffed to be attractive, such as Free Trade, Free Movement, etc, etc. An Isolationist or Autarkic power can shrug the WC off, and even a member should be able to defy. We've already had defying the UN in CivIV, with corresponding penalties.