I like the idea, but the lack of backwards-compatibility is less than ideal. Some people who update CoffeeScript compilers will suddenly find their code mysteriously doing the wrong thing (when they wrote what this suggestion considers "cascading messages", but expect them to not cascade). I have worked on multiple projects that would fall prey to this issue.
The problem is the time it will take people to realize that they need to turn on the "legacy" option. The failures this change could cause could be very difficult to track down.
