In other words, the problem is that null is treated as if its class is a subclass of all classes. All of a sudden, you've introduced a superset of all of the p

In other words, the problem is that null is treated as if its class is a subclass of all classes. All of a sudden, you've introduced a superset of all of the problems with multiple inheritance. (Nulltype inherits from everything!) You've also made it nearly impossible to adhere to the Liskov substitution principle (unless none of your classes can do anything, making all object instances equivalent at runtime). In type theory, a type that is a supertype of all types is called a "top type" and a type that is a subtype of all types is a "bottom type". It's okay to have one or more bottom types in a type system, as long as it's not possible to create instances of them. Unfortunately, in most of the popular statically typed languages, null is an instance of a bottom type. One reasonable use of a bottom type would be to allow a variable to assume one of many types, and require a dynamically-checked upcast when actually passing that variable to a function that cared about its types.