Let me ask you this and I ask not to be clever, but, just out of curiosity. The water coming into your house is treated, bacteria allegedly killed off, etc. Yet, you probably have had to deal with some mold or mildew in at at least your bathroom. Why is that? It certainly isn't the volume of water that is in a PoTC situation. Moisture is also airborne from outside sources that unless hermetically sealed will get in. I think it's a safe bet that Pirates is not hermetically sealed.
Let me be careful to point out, that I am not disagreeing with you that more general upkeep is required on a regular basis, but, also to express that I feel that, over time, nature will catch up and exceed the ability to completely keep up with the best of efforts. Mold spors are airborne also they don't just go away without significant chemical exposure that would also expose passengers on the ride. It's not a simple answer like everyone seems to think is should be.