You may find easier to answer to a direct question:
Since you're the one complaining, the burden is really on you to explain why it is _not_ a good idea. But I have an answer anyway:
Why do you want the tearability of a nullable value type, which is an implementation detail, to be part of the type system ?
Because it is not an implementation detail. It is a semantic property of the class that describes how instances may react under race. Whether or not a marker interface is the best way to do this or not, this is 100% fair game for representing in the type system.
Got any other arguments?
