Why doesn't generic ICollection implement IReadOnlyCollection in .NET 4.5?

Question

In .NET 4.5 / C# 5, IReadOnlyCollection is declared with a Count property:

public interface IReadOnlyCollection

        

Answer 1

It would be semantically wrong, because obviously, not every ICollection is read-only.

That said, they could have called the interface IReadableCollection, while an implementation could be called ReadOnlyCollection.

However, they didn't go that route. Why? I saw a BCL team member write that they didn't want the collections API to become too convoluted. (Although it already is, frankly.)