Why is DefaultSentenceBuilder private?

Is there a reason that `DefaultSentenceBuilder` is marked `private`? It would be much easier to customize `SentenceBuilder` if the class were public and could be inherited.
```csharp
public class MySentenceBuilder: DefaultSentenceBuilder
{
public override Func UsageHeadingText => () => "Usage:";
}

SentenceBuilder.Factory = () => new MySentenceBuilder();
```

Another possibility would be to make `SentenceBuilder` non-abstract, move the default implementations there from `DefaultSentenceBuilder`, and then get rid of `DefaultSentenceBuilder` altogether.