The Last Supper has provided employment for conservators for hundreds of years. He used a new method and experimental materials and worked in an unstable environment. The stream running under the wall provided a constant source of moisture - a great facilitator of chemical change. The only other thing he could have done to sweeten conservators' pension plans would have been to install skylights. Leonardo was a creative person, not content with tradition.
Conservators are, well, conservative .
They want things to remain unchanged. The materials and techniques that conservators use are rigorously tested, but it is difficult to determine what will last forever. The best proof of long-term stability is a history of longevity. Materials that have remained unchanged for the past 500 years are more likely to survive the next 500. Environments that have promoted preservation for the past 1,000 years are a good bet for the next millennium.