Why is PersistentModel a protocol and not a class?

PersistentModel is a protocol but would make more sense as a class that we then subclass. If all of the implementation was in a parent class of our model classes then there wouldn't be all the problems caused by requiring the use of the @Model macro, e.g. default property values not working, unable to subclass, overriding get/set not possible, conflict in property names...