Why doesn't `IList<T>` inherit from `IReadOnlyList<T>`?

Question

@w.b put a link to New interfaces IReadOnlyList and IReadOnlyDictionary in the comments that contains an answer:

Why did we not change the existing interfaces to extend the read-only interfaces?

It looks like a reasonable assumption that it works because the read-only interfaces are purely a subset of the read-write interfaces. Unfortunately, it is incompatible because at the metadata level every method on every interface has its own slot (which makes explicit interface implementations work).

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Immo Landwerth | .NET Framework Team (BCL) | http://blogs.msdn.com/b/bclteam/

To explain this a bit more clearly:

Suppose that a program written for .NET 4.0 contains a class MyList<T> that implements IList<T>. It clearly cannot implement IReadOnlyList<T> as that interface doesn't exist.

Now suppose the system administrator installs .NET 4.5 and suppose that .NET 4.5 made IList<T> implement IReadOnlyList<T>.

If the program would then be loaded, the runtime would detect that MyList<T> claims to implement IList<T>, but doesn't actually implement all the methods: it doesn't implement IReadOnlyList<T>'s methods. The program would no longer work.

The C# compiler might be able to match the methods by name, but the runtime doesn't do this. Since .NET 4.5 was supposed to have backwards binary compatibility, interfaces couldn't be extended to implement other interfaces, not even if those other interfaces contain a strict subset of the required methods.